1) They should be close, everything else at this point is speculation. May vary depending on stuff.
2) There isn't a consensus on that as far as I know... so let's not even talk about a clear breakpoint.
3) Imp doesn't require much control and its dps is really close to others so that may be why they use it.
Really until we get a lot more raid data you wont get a clear answer to these questions now. Between aff and demo, just play the one you play best, and between GoSac and GoSup just ask yourself if you're okay with microing your pet.