I'm assuming your referring to a cantilever on a flat top AR here. My Aimpoint M2 on a GG&G cantilever mount cowitnesses with the GG&G BUIS's I have on the gun. If its not dead center, its very close to it.
Now the M2's on my AK's do cowitness in the lower third of the tube using a low Aimpoint ring when mounted on an Ultimak rail.
Personally, I'd skip the power and stay with the standard sight with the Aimpoints. If you want power capabilities, there are scopes out there that will fill the bill and still allow for fairly quick close range shooting. I've been eying up one of the 1x4 Trijicon Accu Points for one of my AR's. It already has a Leupold 1.5x5 on it and it works well, but no red dot, or night time capabilities.