From another forum:

Quote:
OK here's the update as told to me and quoted from the MDNR Facebook which can certainly be regarded as public info. The DNR representative responding is Debbie Munson Badini. When I asked if this was the official MDNR position she responded:
Michigan Department of Natural Resources
Ted -- yes, this is the official DNR response. If you have further technical questions, I'd be happy to put you in touch with someone from our crossbow committee. So here's it is:

Michigan Department of Natural Resources
Ted -- the entire combination (crossbow plus bolt) needs to be in compliance with the 350 fps capacity rule. Very few crossbow packages are available that exceed the 350 fps bolt velocity regulation. If you are making modifications to your crossbow or accessory equipment combos, the DNR recommends that you consult with your equipment supplier to ensure that you stay in compliance with the 350 fps regulation. Law enforcement will evaluate potential violations on a case by case basis. Thanks again for the great question. Unquote


I believe this is pretty clear from this statement that the crossbow will not be deemed illegal UNLESS it exceeds 350fps with the bolt you are using to hunt with.