Stock for stock, I suppose on paper it's possible that BMW has a slight advantage, however I'm of the strong opinion of that's where it ends. So far in it's most potent form, the late model 'uz-fe motors have made a consistant 17-1800HP, and while it's apples to oranges in to a street driven setup, may of the components are the same. The potential of the 'uz-fe is a very high bar to meet.
As for the Bmw V10, it's a nice motor for a one off, but let's be honest here.. a one off may be about as far as it goes. There's a lot of questions I'd have about the block and heads as far as performance is concerned. Way too many to list here all at once. But having developed/in developement of a V8 swap for the Supra, I can honestly say that finding the core motor is only about 5% of the battle. Making it work is where the real money comes in. Fwiw, the Bmw V10 is being advertised at 8KGBP, and for that price you can have a similar displacement V8 built well, and ready to drop into the Supra. Add 3800GBP and you can have a twin turbo kit that'll kick the crap out most anything on the road.
Possibly the V160 can be used, and that goes into what I was talking about before. To do it right, you need the exact face of the V160 and BMW motor, then produce an adapter plate. I suppose an adapter could be whipped up but the trans would never be 100% true. The inital costs to do it right could run into the thousands. Then what is the crank offset? The 'uz-fe is lucky because all of the numbers jive, but in the case of the BMW, will the input needle go into the bearing at all? or go in too far? Then there's the issue of the clutch. The OEM clutch is useless unless by some miracle the hub is a V160 pattern. A custom clutch can be tacked on as a developement cost.
I want to say though that I'm all for swaps (obviously ), but it's important not to get lost in the monitary aspect of it. The costs are staggeringly high to do a swap properly. Never in the MKIV can a motor just be purchased and dropped in. It takes loads of time, loads of effort, and loads of cash to get it all working right. From a customers point of view though, and lucky enough, all the the developement costs are immaterial. Customers want something that arrives on the pallet and is ready to rock n' roll.
Thanks!
Eric