i just took off my cats by replacing them with a 2 1/2 inch h-pipe from flowtech. it sounds pretty good, but i noticed a HUGE drop in power. especially acceleration. the headers are still stock (gotta find the time) and i have a flowmaster aftercat system. any guesses?
I have always noticed that a big off road pipe tends to kill some low end. Especially on a automatic car for some reason. I think its a backpressure thing.
You may have lost a little low end torque by going with larger piping but that would have been made up in top end hp. You should have actually picked up some power though. Check to see that you don't have a leak or some other problem that occurred coincidentally with the new H pipe.