Mugen 4-1 is overpriced...if you're going to spend that much money go with a custom Hytech setup...

If I'm not mistaken the all-motor crowd is using 2.5 piping all the way to the back versus 2.25...

Shingo

 

It depends on the motor. If you're shooting for the 190-220 whp range then a 2.5" system is a good idea. Once you get past 230 whp the powerband becomes so peaky that gains can be seen up top from 2.75" pipes but that is obviously at the sacrifice of some grunt lower down in the powerband. You can hit 240 whp with a 2.5" system so to me personally the 2.75" pipes aren't worth it unless it's a dedicated race motor looking for the absolute maximum top-end power.

Hytech and SMSP are essentially custom one-off pieces made to order. You call em up, tell em what motor, what car, etc etc and they'll make you a header. I know Hytech also makes full exhausts and I think SMSP does also.