You can't afford to go that fast.
Actually, I have heard of guys getting performance rebuilds on factory ZF slushboxes with improved bands, clutches, sprags, and whatever the hell else keeps an automatic from grenading. The problem is cost. A ZF reman runs $2k + and a performance slushbox will run at least double that if you can find anyone who will stand behind their work. Don't forget about a larger transmission cooler too. If you want a higher stall speed, plan on spending $1k + for a custom torque converter.