I've been running GTI Cup recently and we've been averaging about 6 car fields with very similar modifications. It's a spec class but we've been playing with the various specifications as a group to make our cars more fun (basically make them faster!)

Parts that are the same include
Rabbit or GTI chassis A1
Same engine prep 1.8 IT type
Same Transmission
Same shocks (Bilstein Sports)
Same spring rates (~500F/350R)
same sway bars etc. (I don't run a front bar)
Toyo RA1 tires

Some of us run a header and a couple of us have Quaifes.

The interesting thing is that the racing has been close and we are unable to notice a difference in handling or performance when comparing the open diff and Quaife cars. Headers don't seem to make much of a difference either as we race down the long TH straights. Here is a link to a pdf file showing lap times with Bailey and Suarez having Quaifes in their cars.
http://home.infostations.net/GTICup/GTI-Cu...mber%202005.pdf

Driver ability is pretty close and Neville has recently stepped it up and competed GTI to GTI with his open diff which negates the theory that my car is the only fast open diff car.
One might conclude that Thunderhill doesn't require a limited slip but Infineon does. I don't know but we will have more data next event since it will be at Infineon.

BTW, the times are decent and compare reasonably well with competitive ITB cars, especially if one adjusts that GTI Cup is on Toyos rather than Hoosiers.