Does the latest lotus subframe not have a bar welded in along the bottom of the two tower parts? Kind of like a welded in brace bar (hope that makes sense). The one I saw was part of the subframe and was about 2" wide by 1" high and made up of a folded sheet spot welded in.
Summat like this:
I'm not trying to rubbish the top work by spitfire (and it is top work!!) I'm trying to assess the need for the movement in some directions
It's good to question!
Briefly, 
Two points, firstly when comparing brace possibilities you cannot compare the Toyo subframe above with the VX subframe, as you mentioned this is fabricated with an RSJ running along the back.
Secondly but more importantly, this subframe still flexes.
As with all complex structures it was designed to flex and it has to do to avoid stress fractures or premature overloading of fasteners due to raised peak loads. A good way to look at structures is to imagine they are made of rubber, this will help you to visualise what will move if pushed or pulled at a certain point. Which bits are stretched, compressed twisted, etc.
All the forces are taken up by the entire structure.
Designing in the flexibility required to ensure several million stress cycles is a complex calculation taking in a very large number of factors, material, structure form, loading pints, support points, just to start with.
Adding bits to a pre fabricated design is also complex if done correctly, I cannot imagine anyone on this forum has either the time or resources to do a recalculation on this subframe so you have to be as sensible as you can and for our thinking at Spitfire this means to sort what needs sorting and leave the rest alone especially is this includes introducing lever moments, shear moments and torque moments onto a non rotating fixing.
The only known BIG issue with the VX subframe is the compression of the rear section, this can be sorted with a brace.
Our philosophy is not to chance stress fractures occurring elsewhere or to (ever) have a rotating section under the head of the major fastener.
The second point will cause an issue sooner or later as the loaded components must wear on each other and in doing so the torque of the fastener will be reduced. Lotus used a bush on the Toyo subframe by the way.
We designed the VX brace over 10 years ago and have over 150 in service.
Never seen or heard of any primary or secondary damage caused to the subframe nor any handling issues.
The feedback on the brace on track has only been positive.
Hope this helps
Gaz
