Indeed some people think wider spacers "square-off" the look. I just still don't see it. I did 10mm on the front and I think that was just enough. Or maybe 12mm as you did as the max. It's easy to "overdo" the fronts with 15mm and make them stick out further than the rears and it looks weird to my eyes.
I really think it's the rears that need more help with spacing than the fronts. Most people who look at the M5 don't realize it's a staggered setup stock. The rear tires are so nice and beefy even as stock size but they're hiding in those wheel-wells. Why not show them off?
Some people on the forum may think having a 5mm difference from front to back may look disproportionate or extreme but I would say it's just a subtle effect. I wasn't sure either but took a chance. I totally love the look. It pretty much looks exactly like this on my car:
https://www.n54tech.com/forums/showthread.php?t=24570
I know the OP was asking about +1 tire size with HAS. (sorry to hijack your thread) When it comes to 15mm on the rear with that setup, I'm not too sure. But as you can see by the pics the tires do sit very flush with the fender so I'd say it would be a real close call. Seen plenty of members do 10 and 12mm without a problem though.