Welcome to the rod bearer club we hold our meetings every 50k to 100k kms, preferably nearer to 50k. As usually beyond 100k kms you get promoted to the connecting rod club.
Yes the M button changes a bit, depending on your setting. Usually the P500S (not P500) will make the throttle response much sharper. The throttle body will now open 100% instead of 80-90% or something like that. The intake sound will be more apparent, that is what you actually hear more and not the exhaust on P500S. EDC will go level 2, to be honest that is too stiff for the public roads so my M button is only set level 1. It would also put the steering I believe on a stiff setting (not sure, I maybe wrong but I do feel it). DSC will go to M Dynamic Mode (in theory car will slip, but assist enough). The archaic HUD changes to rpm (not really useful, seems slow and better to look at the real tach) but looks fun. It all depends on what you have M button set to, these are the supposed defaults.... oh yeah I forgot (this feature goes old after 1 time use) the seat bolster will move (delayed) when you turn on M mode.... I feel this is unsafe feature. Imagine turning on M-mode for the first time, go into a corner first time... and suddenly the seats move?!?
I wish there as a P400S or something, 500 HP on public roads is over kill and not as enjoyable as lower powered car. This also means less time on the upper range of the revs (it becomes a different car past 6,000 rpm).
Yes I have never used S6, I feel really bad when the car is already on S5. I have now learned to slightly back-off after the up shift, just before it engages the next gear and step on it after the gear is engaged. Significantly makes the shift smoother in S3 to S5, and makes driving more engaging. Another trick I learned from an owner at Christchurch, is just use the auto S1 (default, was super annoyed by this before) for the warm up. It's great for this use case, as it's super aggressive in shifting up engine hardly touches 2,000 rpm.
Hope to see photos of your car, E61 is not something we usually see often.