I have extension boxes on my 386 Sonic with XR's. Powered by 575 SCi's and it eats the XR's at least once a year. I was told that I would have better luck with the drives without the boxes. From what I understand, the upgraded drives still use Mercs gears.
The Powerplay may not be as heavy as my Sonic but with that hp it won't take him long to realize that he made two mistakes. One extension boxes, the other XR drives.