ShinshikiR8 LMS seems tempting as long as I don't have to add engine tunes or turbos.
Well of course different slicks are different. But having driven Skip Barber open wheel cars on a pretty soft BFGoodrich compound I can tell you the softs in GT5 are too grippy.
You can try to replicate real life times on GT5 but the real world dimensions and physics will never be replicated in GT5.
Edit: Final details are being worked out as we speak. Sign up thread will be up soon.
EDIT 2: OP has been updated with new cars, specs, times, schedule, and rules.