Well just in case the x5 3.0, 4.8i, & X5M all use the same ATC 700. 3 series use the ATC 300. Each cost about 2400 new so maybe the guts of the 700 swap? Anyway I could not find any TQ specs on either but the 700 is clearly good for some massive amounts, but they put the same unit in the 3.0 as the M....so maybe the 300 is overly stout as well.
I keep thinking that in a very real way the TC only needs to send at most 40% to half the TQ to the front end so even with a single turbo vishnu kit that is only 200-250 or so ft lbs max. The rear end will take most of the load and that is via a straight drive shaft right through the case. IF BMW spec'd the TC to send ALL the TQ of the stock motor to the front then it ought to be good under those conditions.
But then it is the axles that are the weak link in the rear (after an LSD). So in a weird way the TC may make needing super rear axles unnecessary (since a good part of the load is taken by the front). It may be that little flimsy front drive shaft needs to be beefed up, or even the front diff/axles. I am sure someone will find this out the hard way soon.