[atmosphericM]

Ok well here are my two cents:

1. It will sound like crap. Mufflers refine the tone. But if this is purely for when you are racing, then ok. To each his own I suppose, but there is a point when you are seriously infringing upon the comfort of those around you. When a modded Harley drives by and I literally have to plug my ears for them to not hurt, I roll my eyes so hard. I mean wtf, really. Some places even have laws against this, with good reason.

2. Yes it will probably give you about 5 whp to bypass the muffler. But you'll also hurt your hp a bit when it's closed because of the turbulence it will cause in the exhaust stream. Also, a tune is becoming more and more necessary to fully utilize your exhaust mods with each one you do. Personally I wouldn't even run headers without an aftermarket tune as they're already probably affecting the AFR's (not sure how much our DME's can "adjust" to engine air mods). With catless headers, catless front pipes, and an open cutout, the air flow is so much freer and different than stock, a tune is really needed to enjoy the benefits. Without a tune, some top-end HP gains may be happening, but almost certainly at the expense of low-end TQ.

3. I could be wrong but I'm thinking that there is only one place you can even put the cut-out, which will require a different mid-pipe (y-pipe), because in stock form, the resonator is in this location. The rest of the exhaust "area" under the car is pretty tight... but perhaps you could find another location... maybe.

4. Would it sound like stock with it closed? No. You already have headers, your car already doesn't sound like stock. If you delete the secondary cats, your car is going to get louder and raspier. It wont be super loud or raspy, thanks to your stock muffler, but it will be noticeably different than stock when the motor is under load. With a cut-out it will be deafening and, to anyone above the age of 25, annoying AF.

5. There is at least one good YouTube video of a guy with a cutout on his 328i. That should give you an idea of sound.