If you look at my avatar, you'll know I like rotary engines. That said, I've gradually come to obsess with RX-7s since GT1 when I got sick of the Viper. I'd call myself an expert on RX-7s but I know there are people much better than I out there.
First off, I turn off TCS and ASM. This means you need to be easy on the gas, even in stock with the '90s models. It's a light car with 276bhp around 1200kg depending on the model. All of the 90s models anyway. Second, the stock Spirit-R and probably all the others, have soft suspension compared to the GT3 counterparts. It may be light, but by jerking it around, you'll be transferring weight all over the place.
So as far as driving the RX-7, go easy on it. Careful turning and controlled acceleration/deceleration. The less input, the better as far as cornering and braking. With the gas, don't hit it or floor it, ease into it.
As far as set-ups, I've found with full racing suspension in the first day of owning the game, lowering it all the way with springs stiffened by 1.0 each front and rear and a slight increase in camber drastically increased my turning performance. However, watch out for them rumble strips and curbs. Softening the shocks did not help me here (bound or rebound) so experiment a bit. I'll do the same and repost a better set up.
If you have a lot of power, a positive toe setting on the rear should help out. But the most important thing is to go easy!