It will handle the power fine. You didn't mention what size tires/gears you have so this is a vague answer. But if you are running 33's or less and 3.31's or better for gears then you aren't working the trans too hard. Get a cooler (if you don't have one) and keep the fluid changed on a regular basis and this trans will serve you well.
If its worth anything, Ive got a buddy.. Tony Carricaburu, who writes the tech articles for off road.com's jeep section.. his jeep is runnin 38.5" swampers, with 4.10 (soon to be 5.13) gears, a 4.2 with a 4.0 head and mopar efi.. and his swapped in 999 is holding up just fine.. I say go for it...
My poor 999 did hold up fine until it got its 3rd internal mud/water bath. Actually it still worked fine after the 3rd flushing but i had it rebuilt and the vent relocated some time after that. I got tired of flushing it.
Can someone please bring me a hammer my 33's just ate my fender again. Damn i need more lift.