Whatever minimal difference there is from upgrading to a slightly lighter muffler is offset by the fact that the aftermarket one doesnt have that bubble shape. The big bubbly shape of the OEM exhaust helps extract air from underneath the car and improve aerodynamic efficiency. Plus, the OEM muffler is a really good design (it has a hemholtz resonator in it which flows well while filtering out unwanted sounds/droning.
7hp from a new muffler? s***... I wish it were true. When I deleted the secondary cats and upgraded the very restrictive muffler on my BMW I swear I gained no more than 4-5hp and that was mostly near redline. Expect a 1hp difference wth a new muffler at the most.
The aftermarket one, is most likely lighter as well, so you can gain a bit there are well.