It's not so much about HP, it's about torque and the transmission. People have certainly discussed transmission limits (somewhere between 350 ft-lbs and 400 ft-lbs), but not many people have actually broken them.
The transfer case's clutches are sized to send most of the output to the front, so I would imagine it can handle quite a lot to the front.
The best place to look would be in the N54 tech section. Plenty of stories of broken engines and other mechanicals.
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