I see a bunch of these on eBay and am wondering is anyone has used one of them ar one from a different site before? Results?

 

All it is is a test pipe. High flow cats on ebay are just a pipe. and then you'll throw a CEL and be polluting the environment. IF you really want to use it, youll need to get an O2 simulator from someone like http://www.kteller.com

Youll see the best results with i/h/e combo, but you wont see more than a few hp. and a stinkier exhaust.