I just wanted to add some info for anyone trying to do this in the future:
I have a mid 2012 cMP 5,2 ... I have upgraded the processors to dual Xenon 5690 (3.5GHz) CPUs, 48GB Ram, Nvidia GTX 1080Ti and I just had a hell of a time getting windows 10 onto boot camp onto it.
Here's what worked:
I had a real windows 10 install dvd. No attempts to make USB startup disks worked with my cMP as others have noted.
Eventually, despite successes others have had without removing drives, I had to remove my drives except for one dedicated one for windows in order to get past the windows 10 format / prepare to install. While other drives were present I could not get the win formatter to succeed. Once it finished though (with just one dedicate win drive installed), then it's fine to go back and put your mac drive(s) back in. Its something about the win installer not being able to get to/format the intended drive.
After all was said and done: the hard drives had to be OUT, and just one blank HD just for windows IN. and the dvd installer was the only way to do boot to the installer (most people seem to use extract ISOs of w10 and burn a dvd, but i had an original disc). And after windows boots up, you can run Setup.exe in your copy of the bootcamp helpers folder (download version 5.1.5621 from apple as others have noted here). That, for me, worked fully.
I did start out by editing my plist file to allow my version of bootcamp to create USB disks, but it really was for naught, since the cMP would not boot from that anyway, so don't even bother trying to boot from USB, it simply never worked for me. boot from DVD, install to blank HD, without any other HDs installed.
And good luck!
Tom.