Part of the problem with your street tires is the tread block depth. If you have significant tread depth then camber alone won't completely solve excessive wear of the outer tread blocks. Also, you can get excessive heat buildup in tall tread blocks which results in more lateral deformation and increased wear. That's why race tires, even r-comps, have a lot less tread depth. Shaving most r-comps will result in better lap times because it minimizes what tread squirm there is.
Here's a good article discussing tire pressures:
Another rule-of-thumb for front engine, rear drive cars is (works for all BMWs I've driven on the track): to reduce oversteer you can either increase rear tire pressure or decrease front tire pressure; and to reduce understeer you can either decrease rear tire pressure or increase front tire pressure. The next time you are at the track play around with tire pressures to see the effect it has on handling characteristics - fine tune the handling and braking balance of your car!