Its just not good to run your PSU on its limit although the PSU calcs do calculate max load I think but the MSI one is supposed to be accurate.
So the chances of having all the components on max draw is slim and the 750w could be fine.
If it was me I'd be going 850w but if you've already brought it it is a quality unit so I'd try it, maybe with 1 less drive and 2 less fans.
Yes, some samples of that psu have been able to do 900w. It is a very good unit, and will have no trouble at all with your computer, even with crossfire.
Oh and btw, most psu calcs are just plain wrong