It is just hard to say at this point, with any of these cars. I've yet to see any ITR car (even very well driven ones) go significantly faster than an ITS car, if at all, so there clearly is a lot of work and development left in them. I have yet to see an ITR Supra or 300ZX, or 968, and I think all three could be way fast despite the weight. Same with a Boxster or an S2000.
Too many variables right now to know. The S2000 makes excellent power, near best in class, but probably won't respond very well to IT tweaks and is torque deficient. It kind of looks like the RX8 in many regards. We'll see.
Build one and find out! An expensive proposition in R isn't it?