Cheers for the comments everyone!
Took the car out for its first twisty road test since fitting the new wheels and tyres and I have to say the grip and balance is almost scary!
Front end is really strong now. Maybe not as superglue sticky as the 285 R888s I use to run, but much better than the Dunlops or Bridgestones and light years better than the 255/40 MPSS.
As I said before, it really is like they are a completely different make and model of tyre, not merely a 30mm wider version of the same tyre.
Thankfully the balance is now perfectly neutral rather than understeery. The rear 285s still have no problem coping with the higher g the fronts now allow the car to enter a corner at.
Peterpeter, I will ask the Editor at evo to update the GT-R's laptime and of course, soon there will be two laptimes (Before and After Litchfield suspension kit fitment) from Stealth Zilla in there too, although not in the Knowledge section as it is a modified car.