[atmosphericM]

So does the car in that first video you posted still have the secondary cats? Either way, I can tell you, without a doubt... that car is LOUD.

If I were you I would probably start by deleting the secondary cats, as those are now your biggest power eaters, and then go from there. aFe Race Pipes or SuperSprint Section 1 are the ticket.

Then, if you don't like how the exhaust sounds, I would start working on your own custom setup that I believe will consist of a resonated y-pipe plus a custom muffler/tip setup. I'm tempted to suggest AA Headers -> aFe Race Pipes -> Stock mid-pipe -> BMW PE Muffler (Only), but I still think that would put you in "obvious aftermarket exhaust" territory, thus removing sleeper status. I'm not saying it would be loud, but it wouldn't sound like stock, that's for sure.

IMO, it will never be a "sleeper" if you touch the exhaust anymore. I haven't heard them in person, but I am assuming that the AA headers alone are giving you a bit more growl than stock.. probably to the level of a completely stock 370Z or maybe stock M3? Maybe a bit quieter? I think even race pipes alone are going to make the car louder than most stock cars, thus removing sleeper status. Perhaps leaving the secondaries alone and going with a custom cat-back setup will keep everything relatively quiet, resulting in a "quasi-sleeper" setup, but I doubt that change would give you more than 5 HP max.

The BMW Performance Exhaust (which I'm aware you've already, rightfully, crossed off the list) is actually NOT a "super-quiet" exhaust like so many here claim. I would say it's a 100% average volume aftermarket exhaust system. The muffler might be relatively quiet, but combined with the deletion of the mid-pipe resonator, the sound level is average I'd say. These cars just have so much sound deadening, and the exhaust is so well designed to not drone, that driver's can't really hear it inside. With the windows down and passing traffic or a close-by building, though, it's pretty dang loud.