Yes, you can simply unbolt yours and put the M3 on. My hangars were rusted heavily so I replaced all the hardware. I used all M3 hardware. The hangars are different. They have a metal bracket that probably adds additional support due to the heavier weight of the catback. I'm pretty sure you can use the stock 325i hangars if you want.
The twin exhaust tips are very high quality stainless steel. These really are easy to keep looking good.
The sound is good, but not too loud.
An extra pair of hands would be useful when installing it. It's quite heavy.
I wouldn't count on any HP gains.