How much horsepower is a function of what year car you have.
If you just take the mufflers off, its best that you go with the same size pipe as the rest of the system.
I have the Snake Oyl pipes and they don't bother me at all. Some people are more sensitive to such things as cabin drone.
As far as seat of the pants feel, again it's different for different people. Some people just have more sensitive seats.
Personally, I felt a much better throttle response with the Snake Oyls.
