Here's an alternative option to spending $2500 + on a system or kit. Since I own an automotive shop, this cost me about 2 hours and about $50 in materials.
What I did:
1. Replaced rear middle muffler with pipe, basically no change. I ended up installing a small straight through resonator where muffler used to be for aesthetics.
2. Proceeded to replace the front resonator with pipe only as the rear section is effectively an H pipe. Definitely louder, but still overall on the quieter side and the tone isn't that great as others have experienced
Overall impressions:
1. Louder than stock for sure, and I don't feel bad since it didn't really cost me anything.
2. Not as loud as you'd expect, definitely not as loud as modified M5's get using the same setup.
3. The rear/middle muffler is hollow and basically made no change in sound, and if I knew this going in I would have left it.
4. Start with removing resonator, don't waste time with replacing muffler.
5. The rear corner mufflers are basically straight through and I highly doubt they do much for quieting the system.
6. Rear cat delete may be next, but I fear it still won't sound very good.
For those not willing to sell your kids or organs to buy a system, this is an inexpensive alternative. After doing this test, I honestly don't see how anyone would spend their hard earned money on a kit that's not going to make that much of a difference. I mean, even with the entire exhaust disconnected and only the 4 stock cats in place, it wasn't THAT loud...... your $12,000 system I don't see being louder than having no system at all, but maybe create a different tone.....
Rear muffler replaced with straight through resonator:
Rear muffler replaced with resonator and front resonator replaced with pipe: