I would suggest a Tubby backbox...which I believe is actually the same as an NA one. I had one on my old red SC with a 2bular 200cell cat. Had to take the 2bular off as it blew the packing staight away and was stupidly loud. The OEM rear boxes dont sound that good but noise test at 88db 
Got a tubby backbox on there already, though not sure what condition it is as I know it's already had to have some welding to cure a bit of blow
Your tullet will be 2.5" and mate with 99% of all exhausts out there.
As you know im in the same boat for an exhaust to fit my needs.
I've had a stock system with miltek backbox which was fantastic for track day noise and had a very uneak jet sound on WOT, 84db and Bedford. Bad point is you'll struggle to get much more than 255bhp (mine made 251bhp 214ft/lbs torque). Negative being the stock cat failed and took a trip down to the U-bend and got stuck there. 
I then bought a tullet mani and 200cel cat/down pipe with a tubby 2.5" backbox and U-bend (note whole system was now all 2.5"). Same car made 262bhp 230ft/lbs torque. The power difference was very noticeable, especially the torque. Noise however went up to 93db but the sound wqs raspy and aggressive. Negative being it was getting noticeably louder each trackday and I'm putting that down to the tullet cat more than the backbox.
So now im back for round 3 and dont want to spend a forth time. I want a 3" system for future proofing my car but without the noise that gets you kicked off track. I hear good and bad about both leading competitors but more so on the cats of one type. I know Lee has a good set up that some how got him on Bedford but cant remember which that is. Theres also davemate's custom asbo set up which is 2.5" and gives good power that any stage3 owner would be proud of.
Not much point in a decat or second silencer as neither seem to give their desired effect and cost £££. We need as free flowing a set up to produce the best gains. I think others have been on to something about the trumpet effect from the twin tips. Maybe some testing could confirm this?
I also know DaveHardwick has been approached to build a SC system but this takes time and a test car. Time most wont have or maybe one of us stay at home type's might?
anyway, thats my input. 
Funnily enough I had been thinking about Mr Hardwick. Would be more than happy to use my car as a development car for a new SC exhaust, with whoever wanted to sort it, it's not like I'm doing anything with it myself and probably wont be for a while yet. Have also dragged Davemate's asbo exhaust thread back from the dead in the members products section.
And I think you're right, I want a free-flowing 2.5-inch exhaust that doesn't restrict power but is also semi sensible in terms of noise levels.
As for Lee making it onto bedford he had to bung up one of his exhaust pipes basically and had to stuff all sorts around his air filter to quieten down the induction noise basically. From that point I'm actually thinking a dummy exhaust exit wouldn't be a bad idea, though what would be ideal would be an exhaust flap with actuator. Turn one pipe off for track use to cancel some noise, turn it back on for grins on the road (and turn it off again if stopped by asbo plod).
There is no easy solution, or every tracker would have had that allready.
Your 2.5" tullett exhaust manifold is excellent, and it get's restrictive after that, with 2" oem.
So on your oem cat section, the precat needs to go. (if not done allready). How the 400 cells cat section will hold up with 280; who knows.
The backbox i would indeed replace with the vxt one (make sure to get one INCL!!! the ubend) for optimal quiteness.
Take it from there and see how it works.
Investmentwise you've then done 50 pounds or so (ex labour)
If your 280 horsies setup blows the baffeled vxt, or the cat section to pieces, so be it.
You're then stuck with replacing the baffeled setup with a straight pipe solution, and a less restrictive 200 cells cat..
Those will always be louder, it will get expensive and may get you into trouble at restrictive trackdays.
Welcome to the exaust problem club 
edit: ah i see the monkeymeister blew his cat with a stock stage II.
There you go, 400 cells is to restrictive to survive.
Maybe get a used mitek 200 cells cat, and the vxt backbox on that?
I'm not planning on doing this more than once, so I think rather than cobbling together some pieces of exhaust and seeing what happens I'm going to bite the bullet and come up with something decent from the start, maybe even a bespoke or custom system.
The Tullet manifold will be staying though, and i don't think I'll be moving above 2.5inch diameter with the rest of the system in a bid to keep the noise down. As you say it will handle 280ish bhp anyway.
I've had both the extremes. Ran a Larini for a couple of years; loved it as an NA but found it too much supercharged. Swapped out for a turbo back box which is utterly characterless but at least quiet. Neither are that restrictive. I think I still had the Larini fitted when you drove mine.
I think it was the Larini, and for the road I'd be happy for it to be properly loud - however I want to use this car on track, so will have to make sure it can be quiet when it needs to be.
I have a 3" tullet on mine with a decat silencer. I blew out the initial packing straight away so its key to get the highe spec packing. With a cat in place its quite droney above 50 mph on the road, with the extra silencer it moves the drone up to about 70 mph but its still there. This gets you on track (dunno about bedford). You can go quieter by having a single tailpipe and a dummy second to get round the trumpet effect, I believe this cuts off about 5 db.
I do think a single tailpipe and dummy second might be the way forward (even if it goes against my grain). I know mike pointed his at the ground when in TA also so noise was being directed at the tarmac not directly out the back of the car.
Tullet mani, tullet cat, VXT milltek silencer.
Perfect S2 combination for power (260), torque (230) and noise (93 dab static at 6k rpm), 2.5 in throughout and looks oem from the outside (VXR milltek).
All available for less than £1k if you shop around.
So is it the tullet cats that are better than the 2bular ones for melting then (though I hope my properly calibrated SC shouldn't have that problem anyway). And yes, 2.5inch VXR milltek isn't a bad shout.
Tullet mani, tullet cat, VXT milltek silencer.
Perfect S2 combination for power (260), torque (230) and noise (93 dab static at 6k rpm), 2.5 in throughout and looks oem from the outside (VXR milltek).
All available for less than £1k if you shop around.
Pretty much the above.
2.5s have seen 300bhp so will be enough for your setup
3inch more risk of drone and for some tracks would probably require silencers. Not a biggy but extra cost/faff when a Milltek will put you on track without worry
Motorway with my NA Milltek is fine and gives enough of a sound remind you it's there.
With the smaller pulley when you jump on the loud pedal you'll hear your SC more than your exhaust anyway 
You're right, from inside it'll be the SC that I can hear when on the loud pedal. Just need to make sure the whole setup is track friendly.
Thanks for all the above info guys, really helpful. Even if it's a shame I can't just go to one supplier and get exactly what is needed in a one stop shop.
G
