I did capped mod. in couple of my Head Pro's. It might be easier if you trim the (cap)bumper first when it's not on the frame. I used a heavy duty scissor, it was pretty easy. The scissor cut through the bumper smoothly and I trimed the ends in a V shape just for a nice clean stock look.
It's a little trickier to try and trim the cap bumper when the bumper is installed on the racuqet. It can be done but it takes longer and would scratch your racquet a bit inside the groove but you can't really see it so I guess no biggie. I used a box cutter and the outcome was not as clean and smooth a job as it was with the scissor. One thing to keep in mind is that the bottom piece grommets is a little too short so you might need some tubes.