There has been a lot of threads on cat backs lately so I was hoping to clarify some things?

-Is the Stillen Cat Back a true "cat back" or is it a resonator back?

-Does the Greddy include a new resonator? ie a true cat back?

What are the Dyno gains of either of these systems? I haven't heard anything about the actual power gains...not talking butt dynos but the real thing. I know they make a lot more noise but do they effect either of the dyno curves in any way?

 

I don't think Stillen makes a cat-back for the 5th gen.... only the muffler.

The Greddy is a full cat-back that includes a resonator as far as I know...

 

you are right.
Stillen only offers you a rear-section muffler.
GReddy is a complete cat-back system which includes both the b-pipe and the resonator.

 

You'll get about a 5 hp increase for a full catback system. Most of that increase will occur at high rpms.