I replaced my main cat with a straight through pipe. I didnt get it dyno'd b4/after etc, but I can tell you I felt the gain instantly. I dont know about top end, but I can tell you, you will notice the extra torque down low. I dont think the car lost anywhere from this mod.
I think the stainless pipe cost around $150, and it was the best bang/$ mod I have made to my car.
When I installed the more expensive stainless twin exhaust, I did notice the slight bit extra power, but nothing compared to the cheaper cat replacement pipe. When my car needs to be MOT'd, I just swap the cat pipe back in for the day.
