Like Tys said, it can be a major problem. One tip that Madmax posted here a while ago was instead of filing it to fit the 2011 slide, use a heat gun and stretch it. You would still need to cut/dremel out the trigger guard area as a hi-cappa has a square trigger guard where a 1911 has a rounded one.
I have heard that a Safariland Glock 17 holster will fit a 4.3 hi-cappa decently, really tight but it still fits. As far as I know there are no Safarilands that fit a 5.1 though.