Twin turbos are nice for having power throughout the power band, but for raw horsepower they are very limiting. If you're building up an RX7 or a Supra TT the first thing you'll do it get a larger single turbo to replace the dual turbos. A twin turbo uses the smaller turbo which has a small rotational mass and spools up quicker to sping the second turbo up. The problem is that smaller turbo then limits you overall horsepower. It allows you to keep a nice torquey bottom end.

-Tim