While I haven't owned an RX7, I did drive one, and it's speed seemed fairly comparable to the R, though definitely way more torque down low. It had Koni Shocks and Ground Control springs I think, lowered maybe 1", and it was solid, no bodyroll whatsoever but still comfortable. The brakes were nice too, though I didn't get a chance to try them out like on my R. Some of the things to consider in the decision are reliability and utility. The RX7 has been known to be not the most reliable sports car out there, especially when modified improperly, whereas Rs are pretty damn good in that respect. Also, you'll have trouble transporting much of anything other than one other person in the 7, whereas the R has at least an excuse of a rear seat and a reasonable rear cargo area. That being said, some day when I have the money for building a reliable 13B, I'll be getting an RX7, though probably as an addition to the R, not as a replacement.

One other thought, you referred to the Supra, 3000GT VR4 and 300ZX as "supercars," but not the 7. I'd say given similar mods the 7 will easily hang with the Supra on the straights, and will definitely pound its ass into the ground through the corners. The VR4 is a pig BECAUSE of the awd, and the 300ZX was pretty tame (IMO) in stock form, and had problems with overheating to boot. I'd take the 7 over any of those in a heartbeat.