the stock us turbo motor (3sgte) has a lot of potential. there are a lot of people who have built their internals and put on bigger turbos running well over 300hp to the wheels.
if you have an na, a popular engine swap is to the 3rd gen jspec 3sgte/tranny. i'm not certain which, but this engine came with either the ceramic turbine ct26 or the ct20b, which has higher hp potential (about 300rwhp). you'd need to get a rear clip so you have all the wiring included. there are a lot of people over the on
mr2 board who have done this swap and have better info.