Depends what engine setup you running? The cen pipe really opens the stock K5.9 engine up makes it a totally different engine in tuning and power curve. The ercm pipes really are the best pipes I have ever ran.
Yup, all depends on the engine your using. If your after an over the counter something that can be had at the LHS, then anything with a three spring coupler is better than stock, just make sure you take a good look at it, so you'll know if you need to make your own mounting arm/rod/spring thing. I make mine out of clothes hangers, and fuel line for flair.