If you're fast already, ASM will only hurt your times. It basically works by applying throttle or brakes to keep the car from over/understeering. If you open the tap while driving in anything other than a perfectly straight line, the ASM will usually detect the slightest amount of slip and engage the brakes. Considering the GTR is AWD on comfort softs, I can imagine the ASM would be clobbering the brakes as you get onto the power after the apex. "Clobbering" is probably a bit of an over-exaggeration, but the last thing you want when trying to accelerate is for the brakes to engage at all.
It's one of those things that looks great on paper, but is rubbish in practice.