At what power levels is it necessary? is it over 400 or before that?

 

well, most likely it will be a 2.5 in DP flared into a 3 in. Then 3 in all the way back. Ppl around here say 250 + hp go 3 in all the way.

My question is, there is still that restriction in the 2.5 portion of the DP? Its a bottleneck period. Does it even help much to go 3 in?

 

A 1/4" bottleneck coming out of a 2.5" turbine outlet isn't that much... If it steps up to 3" immediately, there is no bottleneck, really. If you use a 2.5" U bend to clear the A/C, THEN you go to a 3", that's not really helping much, maybe a little, but make sure you've got a 3" straight through exhaust after that to make use of it.

 

What if you have a cutout coming from the 3" portion? Will it make much of a difference over a 2.5" dp w/a cutout?