I read this very informative article
on Dinan's site that talks about suspension travel. It states you need a shorter bump stop for a proper spring perch setup. Cutting it is not the solution. You would need one that's .5" shorter than the stock ones.
I have no idea how tall the stock ones are to begin with but if for instance the M3 bump stop is .5" shorter you should be able to use those. Even the BMW Performance Suspension Kit
comes with new bump stops.