For me, there's no contest: the RX-7 wins by a mile. You already mentioned the car's main problems, and that's about all you have to worry about. Now, there are a few reasons that this might not be the car for you, such as:
- You're big on "low maintenance" cars
- You won't dish out for octane boost
- You can't afford to spend time and money in the shop
- You really need good gas mileage
Remember the basic rules of modding for any car, and apply those to the RX-7: intake, exhaust, and engine management.
The twin turbo 13B gives you a lot of bang for your buck, but you really need to work it over. Some not-too-radical porting is where you should start. Next, get over the cost and buy ceramic apex seals. You
need them if you plan on using this car competitively, that's including street racing. The fact is, the RX-7 runs really hot, and unfortunately, one ping will destroy a rotary. The ceramic seals will make it a little more detonation resistant, and generally increase the lifespan of the engine as well. If you're lucky enough to find an R1, you'll already have twin oil coolers, but otherwise that should be your next investment. Underdrive pulleys make cheap (almost free) power, and they're not very expensive either. Finally, a single, ball-bearing turbo is the best solution for power, efficiency, and reliability. You can hold off on this until you have problems with the stock TTs, or until you're ready for all the extra power.
Now, if all you're interested in is straightline performance, then you can throw the 50:50 weight balance out the window and drop a triple-rotor 20B in. I think the advantage to this is self-explanatory.
Of course, you may have a thing for AWD or something, in which case this is all irrelevant. However, I suspect that you'll find the RX-7 a lot more
fun than the GSX, which is what really matters in the end.