I stayed with stock pipe. It's correct to say that the motor needs back pressure. Not having a cat for some years and then putting one in, the increase in power was substantial. My mechanic said that there is very little difference between the high flow and standard cat, so I went with standard.
I have to put the heat shield back on, it gets mighty warm in the engine bay. As I have no plastic intake up to the air filter box, sucking in that hot air, I have read, decreases performance. Apart from that the main wiring harness comes out of the firewall right beside it. Only problem with putting the heat shield back on is I am going to have to spot weld a couple of bolts onto the top of the extractors. <img src="/forums/images/graemlins/frown.gif" alt="" />