You're probably driving it too hard. I noted that the car likes to turn under braking, and do it well, but too much speed makes for what feels like understeer. When that comes in, I do a quick lift, tap brakes, hit throttle procedure that brings it back in line.
And I got some more seat time in my "Evil" tune for my BTR, swapped over to Blackbird, and it became what it SHOULD have felt like the first time: Awesome. It's GLUED to the road surface whenever you're on the throttle. Off throttle is not bad, with just a little oversteer. To learn this car, a much harder-edged setup is useful, as once you get used to that, this setup makes the BTR compliant for a 700hp 964.
I tried the R4 at Laguna Seca and was a little surprised. I'll agree with Rotary on the car wanting to turn while braking, on most corners I would just tap the brakes 2 - 3 times, 1 at entrance to turn the car into the corner, 1 at apex to once again line it back up, and sometimes a third time to avoid the dirt on the exit. I was running around 1'48's in it.