Changed my M6 thermostat yesterday.
I was throwing a P0128 error, which is a low coolant temp error. When I took the car on the highway on cold days, the error would light up because of the increased radiator airflow, and because the thermostat was open when it should not have been. (Mine was not stuck upen, but was opening at about 60C according to KTMP.)
At any rate, the swap went pretty much as above, with a few caveats. (This was on '06 M6)
1) Drain the coolant first.
2) Then remove both airboxes. Also, remove the lower half of the drivers side airbox.
3) Remove the bolts on the expansion tank. This will allow you to push the now-empty expansion tank to the side where the airbox is supposed to fit.
4) The most frustrating part about the install was dealing with the intake plenum. You have to loosen 5 screws on 5 hose clamps per side to lift up the plenum. Getting to the back screws was challenging. As the previous author stated, you don't have to remove the plenum completely.
5) Remove the hoses from the thermostat housing. Remove the thermostat housing.
6) Reassemble, with 4 new rubber seals, 2 new crush washers, and the new thermostat. Lube the rubber seals with coolant.
7) Make very sure that when you tighten the thermostat housing, it goes on straight, and not crooked. I hand tightened the 3 main bolts, and advanced each one a quarter turn to prevent the housing from tilting. Reinstall the hoses. (I left the Banjo bolt attached but loose, and when I refilled with coolant, I used it as an air bleed. When coolant came through the banjo bolt, I tightened it.)
8) Reinstall the intake plenum. Very frustrating dealing with those hidden screws.
9) Put everything else back together.
10) Refill with new 50/50 coolant. Mine took about a gallon and a half of 50/50.
11) Check for leaks.
12) Enjoy warm air from ventillation system on cold days.
By the way, if anyone needs some rubber O-rings, I have a bunch left over. The place I ordered my parts made me order 10 of them because they didn't come in a smaller quantity. So I have 6 left over. If you want them, message me and I will mail them out.