Is there any disadvantage to the 750W? The RM550W and RM750W in the UK are only £5 different at the moment.
As said, you will loose efficiency at low loads: so higher power draw from the wall, and slightly higher thermal rise inside your case (which *might* rise your case fans speed).
On the other hand, you will gain a better build quality.
Does that look okay?
As said, it seems not very flexible, so probably it might be not enough at the higher loads: if I'm not wrong you will have a minimum 45% PWM at 60°C, which is a bit high (IMO/IME): if your fans works well at 45% modulation it still may be fine, but a lots of fans run too much fast at that value, and so more noisy than needed.
To give you an example, I run mine at about 30% PWM @ 75°C with a Core i7 2600K, about 550rmp, and it is absolutely fine (they handle 8 instances of Prime95 without cranking up).
At any rate, if - as said by CA Steve - the bundled utility let you a fine regulation for fans speeds, and you will use it, then I think it might still work.