It isn't just wear in the trans its the entire drive train that should be considered. There can be no question everything gets worked harder in higher S modes so the real question becomes does it make any significant difference to the lifespan of any of the effected parts? I don't have an answer but one thing that is telling to me is that they made S6 harder to access and that wasn't done without reason. If you start thinking about what that reason might be its hard not to put drive train issues or failure before the warranty expires at the top of the list.
Another thought is that it may not be the shifting level so much as driving style when at that level. It doesn't take long to figure out the aggressiveness of the shifting needs to match the way you are driving or else you end up with some rather unseemly clunkiness of the kind that you know isn't good. An example would be when stuck at low speed in traffic and you are shifting at low rpm and little load. I can get smooth shifts up to S4 in this condition but in S5 its hard to avoid the trans clunking into the next gear when I shift at times On the other hand I find S5 shifts fine with a hard but not clunky shift or bang when I'm driving in a more aggressive manner on the open road. You can pretty much figure there are people who bought these things and put it into the highest available settings in M drive and then keep the car in that mode even though they are not actually driving in a style that would be suited to the harder shifting.
2012 E92 Space Grey with fox red/black/black, comp, premium, DCT, premium sound, sirius, heated seats.