Mine's sharp with the filter on (LX). Yes, it fouls the mirror on M's (took a tiny chip out my Program A (Program Plus) mirror :-( You can grind down the outer edge of the filter ring at a 45 degree angle to make it clear as they're quite thick pieces of ally.
I did this, but no luck - the mirror fouls on the edge of the filter glass. Obviously there's some variation between lens samples... :-)
S
