I've got 18 inch wheel on my car (all 4 corners) which were deliberately wide to give me more traction in the dry.
Front are: 8.5J - 245mm x 35% profile, ET15 with 16mm spacer
Rear are: 10J - 285mm x 30% profile, ET15
Be warned, if you go that wide you will need wheel arch extensions for sure. They certainly make a difference to the way the car handles/behaves, both positive and negative.
Positives:
1. Immense grip on good/moderate tarmac (sustained corners at 1.5g with semi-slicks).
2. Much more security when you boot it, so that the rears don't lose grip.
3. Possibly the car feels more planted, but this might be due to other mods.
Negatives:
1. The car is much more camber sensitive, the steering wheel is very twitchy (even snatchy at times). This is really noticeable and you have to be much more aware of crossing the centre of roads where the camber changes.
2. The reduced tyre profile makes the ride harsher, will probably wear out the coilovers faster.
3. On bumpy road (I.e. pot holed B-road) the car is noticeably less nimble and feels less secure.
4. Marginally worse grip in the wet, massive chance of aqua-planing.
5. If your overall new wheel + tyre diameter is larger than OEM (which it likely will be) then this will adversely affect the wishbone angles.
Overall, I'd say the 18" are worth it if you intend drive on flowing A-roads or not too twisty tracks. If you were trying to belt down a twisty B-road or a track with lots of moderate to sharpish corners you'd be better off with smaller wheels IMO, as they would give a nimbler feel.
This info is as unbiased as I can be BTW.
Edited by Nev, 14 November 2014 - 08:19 PM.