It's all about the tuning. A good tuner who knows what he's doing will get your cold start, idle and fuel economy as good as a factory engine. A crappy tuner will make your car run like, well, crap. The computer used doesn't make much difference in the end, it's all about the tune!
Amen brother
Celica-sue: You can't just look at the raw price of the computer, some computers may require changing sensors etc. which could become expensive.