I've autocrossed with TC on (by mistake) and it really hurts your time. The only exception is autocossing in the rain, then only STIs and EVOs do better than the XFire.

If you are interested in in loosening up the rear, I ran with A6s up front and R6s in the rear to generate more rotation in the back. You can also achieve this by running higher pressure in the rear (>40psi).

Ultimately it is your call if having ABS always on is worth some TC intervention.