from what i've gathered reading on here and other sites, from tuners to experienced individuals (Mr Paddle.Shift, MarvelPhx amongst others), keeping the power output at about ~300 hp and torque at ~295 lbs/ft (or lower) is the general consensus if you want to keep your transmission for a while. this power is achieved with a 5.5 or 6 psi pulley. there are certain ZF transmission models that handle high power better than others, Alpina uses one such ZF transmission in one of their cars (don't remember the exact transmission model - Mr Paddle.Shift does).
then there are members who have 330s @ 6.5 psi producing 330 hp (Shinobi) who has had no problems this far, and others with the same kit but headers and/or cams in addition who's had their transmissions fail.
personally i'd go with 300 hp to play it safe(r) - i put "safer" because there are no guarantees.
"if you wanna be king of the ring, this is the kind of car you should get" -M3 CSL
"there's no replacement for displacement" - only BMW ///M