Ideally, a full M5 exhaust would be nice...I've seen them go for $1-2K plus shipping. Plus, as you might already know, you'd have to modify your rear facia (or go with the M5 facia) for the tips to fit.
Compromise with M6, yes...but the M6 muffler retro isn't too bad. It HAS an attitude for sure. Forum members have been selling used set ups for $5-600 range. Some might already be fitted with quad tips and hangers to fit right up. It is not a bold on but will fit relatively easily with new hangers and a weld-on transition pipe.
The downsize to 2.5 inch a restriction? IMHO, is no big deal at this stage of the exhaust (gases expanded and velocities slower) since they've been compromised already with the numerous crush/kinks already on the factory pipe (check it out next time your car's up on the rack). So unless your dead serious about max exhaust efficiency (upgraded downpipes, racing cats, 3" manderel bent pips all the way, resonator delete, X pipe, and operate the engine in its higher output band, I would'nt think you'd notice it on the butt dyno. I did mine for cosmetics and that "attitude". Didn't notice any performance drop (but in sport + WOT, its hard to concentrate on anything else but straight ahead and you MPH).
The 550i does have a vacuum controlled flap on the inboard pipe of the stock exhaust which is there for sound reduction at start up and low speed (<1500) only. No performance or warranty issue if the vacuum line was disconnected and blocked (lots of post on this).
The sound is definitely noticeable, at start up, off-idle low speed, and especially WOT from a stop.
At cruise or low speeds, minor modulations of the throttle are now very audible. Some people might call it drone, my car buddies call it "hot", I think is sounds like BRRRRRR.
Its actually too LOUD for me and I don't particularly like the attention it draws. You know, the guys in other brand cars that think they can lure you into some kind of macho contest. I'm just too old for that and spoiled by use to my music and quiet cabin. I may end up going back to stock after all.
Lastly, if you hit WOT from highway cruise, sound is not much more aggressive, surprisingly.
Bottom line for me? Go with the full M5 exhaust, M5 facia, and AR downpipes. Then the car will sound AND go like crazy. Throw a din an Stage 2 or three, they is is sick and crazy. Just cosmetics with a bit attitude? buy a USED M6 muffler set with custom quad tips and hangers already welded on.