One easy way, if you are going to pull the two goalrods out, is to just get a "Dremel" or similar handy sander to just burr off 2 mm or so of the bumpers. That way you wouldn't even have to buy new bumpers. I seriously doubt you would ever have to break and replace those bumpers anyway. Only 2bar, 3bar and 5bar bumpers ever really get worn enough to crack or break.
Once you have them off, get a ruler, mark a line and shave off the excess. This and larger, rounder, less hard-rock balls should improve your play, by magnitudes.