My experience (mainly in rural areas of Michigan) has been the MSP is fine up to about 8 or 9 over, then at 10 over will start pulling people over. At 10 over they'll give you a roadside chat, but as long as you behave properly during the stop they'll generally let it go. Beyond that, you're probably getting a ticket.
I normally stick to about 7 or 8 over, allowing some leeway with cruise control to drift up a little on downhills. Beyond that is generally not worth the extra gas and mental stress of watching for the hiding spots the troopers like to use. On the 75 mph sections, I'll let it drift up to 80, but that's about it.
Along the "Seney Stretch" (a 25-mile dead-straight and level stretch of M-28 between Seney and Shingleton), the MSP catches a lot of people off-guard. It's literally the middle of nowhere, and people don't expect actual enforcement there. But there's nowhere to hide, and usually they get a lock on their radar before the driver's aware it's a cop headed in the opposing lane. Usually it's closer to Shingleton, about the point where drivers are lulled into a false sense that there's no enforcement.