This is a late reply but I just saw this thread. I trimmed my strut insulator for the rear as I won't be running camber plates in the rear (only the front).
This is what Ross/Modern Motorsports said in his installation instructions email when I purchased mine from him in regards to trimming that piece and what you use for a bearing:
"2)black washers - 1/4" thick aluminum top perch sits on top of the spring, then anywhere from 1-3 black flat washers sit on top of that silver upper spring perch, the black washers then bear onto the OEM top strut mount.........you can trim that black rubber lip on the strut mount so you only need 1 or 2 washers if you like...(often some metal needs to be trimed to get down to using only one washer, .the more you trim it the more suspension travel you have available. You DO have to use one washer at a minimum for proper safety/bearing so keep that in mind. In the front this washer bears on the isolator right beneath the bearing within the OEM strut top. In the rear this black washer sits against the rubber isolator (no bearing in the rear OEM top)."
I imagine instead of using washers you could buy some needle bearings somewhere from a circle-track racing site or something. Anyone have any reason that would be a bad idea?