Brainiac, I run DOT R compound tires on the track and I can assure you that increasing rear pressure will reduce oversteer, just like with street tires.
The reason for this is that "all else being equal" a reduction in tire pressure will increase the slip angle of the tire. More slip angle rear = More oversteer. More slip angle front = More understeer.
If you would like to see a good table that shows the anticipated effect of various tire pressure and suspension changes will have on your cars handling, check out the following very good ariticle. The table is about 1/4 the way down:
http://www.bmwclub.ca/ccrart3.htm
It also discusses and compares various DOT R compound tires on the market today, including the R1.
The article is written by some BMW club guy (boo hiss) but is chock full of information.
The good thing about tire pressure is that is cheap and easy to change. Try varying pressures, front and rear, on the track and observe the effect it has on the car's handling.