Dude. Get the TT. It is going to blow the rx-8 away in more categories than I care to list.
Is the TT fast? The 225 is quick. Due to weight, the coupe is about half a second quicker to 60mph than the roadster. Roadster 0-60 times are 6.3 or so. Still not a slouch.
Does it handle well? Yes, it handles well, but it is no 911.
Performance mods? Tons, my man. I had mine ('01 225 roadster) REVO tuned (like chipping but better), and it became a different beast. Went from 225hp to 260hp, and 207lbft to 284lbft. She's plenty fast now. I also just ordered a p-flo intake (some debate about these due to sealed engine compartment), a neuspeed 70mm exhaust, and a forge diverter valve. Once fitted I should be somewhere in the 275hp range. I also plan on doing some suspension work (sway bars) within the next year.
Is it reliable? So far, so good. Spent about $250 having some new sway bar bushings put in, but that's it.
In closing, the TT is a great car. It is beautiful to look at (unlike the rx-8), and you really don't see many of them out there (also unlike the rx-8). You can mod it easily, and its a super fun car. If you get one, I doubt you will regret it.