So... In 14 years of existence of these cars no one cut-up a stock muffler?

To truly "feel" power increase, most "normal" drivers need at least 10% of extra power or 20hp in this case. Unless you had a clogged system, you wouldn't see that from a simple muffler swap. A clogged CAT could cause this, but not a muffler.

Also, if you have a significant enough blockage in your exhaust to "feel it", you should see it in your fuel consumption.