I didn't have the HKS exhaust, but I did have the Greddy EVO cb. It is 94mm (3.7 inch) I made very impressive gains over my 3" tanabe exhaust, especially top end. I had stock twins running 13psi.

If the price of the HKS is a problem, I would go with the EVO. I have seen people with single setups get a lot of power out of it as well. I paid $480ish for it from www.rx7.com

 

JIC is full titanium, Amuse is beautiful and actually sounds awsome but very expensive, Greddy has a titanium can for about $600, Rotary extreme may still make their downpipe back titanium exhaust to. Unless you have a 3.5"- 4" downpipe and 25+lbs of boost, what is the purpose of getting 4" exhaust?

 

Those exhausts are hella expensive, I am getting 2 magnaflow 4" mufflers and some 4" SS pipe.