561 Watts at the wall under Furmark. Take 10-20% off that number to account for efficiency and their system would have drawn ~500W, worst case. That would only stress a good 750W PSU to ~66%. Good power supplies, like the Corsair HX 750 are designed to deliver their rated output continuously.
If you didn't already have a PSU it would be hard to argue with recommending an 850W unit, but if you already have a high quality 750W unit, which isn't giving you any problems, then I think there's no need to upgrade.
+1, you are all set with your corsair