It's probably more of a plus than a negative, not having the factory bias valve.
I did read on GTV6.com that there was at least 1 guy who much preferred the car without the factory bias valve. The rear brakes always seem to be under performing any way (at least they did on my 2 Alfa's).
If you are really concerned, then fit a proper adjustable 1 like a Wilwood.
As for locking up the rears in the wet, there is less over all traction available, so the car can better use rear bias because there is less weight transfer onto the front wheels and off the rear wheels.
I did read on GTV6.com that there was at least 1 guy who much preferred the car without the factory bias valve. The rear brakes always seem to be under performing any way (at least they did on my 2 Alfa's).
If you are really concerned, then fit a proper adjustable 1 like a Wilwood.
As for locking up the rears in the wet, there is less over all traction available, so the car can better use rear bias because there is less weight transfer onto the front wheels and off the rear wheels.