actually i find that the 20"+ wheels do not extend past the fender.
with my 19" i was running a 25mm spacer in the back, which is still a lower offset than the OEM 20", a 13mm wider stance.
my 19" were just flush with that spacer.
the reason the factory requires the wider flares is bc BMW wants the wheel recessed by 10mm, like on all of their cars.
its standard practice to have the wheels inset, for safety blah blah.
most people get spacers to make them flush.