Since I put new dampers on the rear of the GTV I have been able to go round corners quite a bit quicker. Trouble is, as many others have stated, is that you end up hanging on to the steering wheel in order to stay in the seat. I did web & forum searches for better seats, but it seems you then have to make custom sub-frames in order to fit them, and the expense.
I went round to a nearby car upholsterer to see if he could suggest a way to improve the lateral support by adding extra padding, but he said that this was not really possible, but by changing the central leather part of the seat to alcantara would make it grippier. He held a piece of alcantara on the seat back to show what he meant, and it certainly felt like an improvement. We talked for a while and because I was loath to have the seats dismantled and the attendant belt pre-tensioners bother, he reluctantly came up with the idea of just having the upper part of the seat back covered in alcantara pushed into place over the leather. He made a temporary cover from an off-cut for me to try, there is plenty of depth available to tuck it in so making a 'permanent' fixing at the top and sides (the alcantara is grippy), should the bottom prove to be a problem that could be 'glued' in place using Sellotape Outdoor Sticky Fixers which are only 0.75mm thick and could easily be removed without damaging the leather seat beneath.
I have tried this out round some fast corners and there is a vast improvement. Not a perfect solution, but with a little practice it is possible to substantially reduce the need to hang on to the steering wheel when cornering rapidly. Not perfect, but a lot better. The upholsterer says £20-30 for a more permanent job on the two seats, so I have arranged to have it done tomorrow.
However if the problem is not with your upper back sliding across the seat, then adding the alcantara will not help at all.
Just need to decide now whether to have black or red. Black I think.