I recently installed a MR45 with 802.3af. I wouldn't recommend it. The best I could get it to work is in a mesh with one AP have 802.3at. I would try to find some cheap compatible injectors.
I don't know. At a guess it may well power down things like the scanning and security radio, may limit the number if MIMO channels (so drop back to 4x4 instead of being 8x8) and things like that.