The tach on the M3 does not indicate actual revs. The M3 cluster was taken straight out of the E60 M5. This is why we see a red line at 8250RPM (the M5 red line). The engines revs are corrected so that when the needle hits an indicated 8250 the engine is actualy spinning at 8400RPM (the M3 red line). You can actually see this by accessing the hidden menu and look at actual revs.
So the OP question is very pertinent. With an increased cut off to 8600RPM, does the needle on the tacho go past 8250 or does an indicated 8250 now correlate to an actual 8600RPM?