Re: I don't see a difference.....
Looking at your card's stats, the factory OC is not very much at all. As you can see from my sig the memory clock can be set much higher. Even with the reference cooler my GPU doesn't get much hotter than 60 degrees. I would overclock more if I were you. Do you have the Catalyst 10.10 driver installed? if not then install it- you should always have the newest drivers for your system.
I've heard that the 5770 doesn't tend to artifact at unstable memory clocks, so when you get an unstable memory OC the performance will decrease. I haven't tried clocking mine too ridiculously fast, so I haven't ever run into this before but I've heard about it. You should be able to bump the core clock to 930 or so pretty easily... don't start there, though- just run it at 10 MHz increments. Start your memory clock testing at around 1300-1350, unless you don't mind spending plenty of time monitoring stability tests.
Don't forget to tweak the performance settings in the Catalyst Control Center- slight driver changes can make a big difference in performance and quality.
AMD FX-8350 Black Edition
ATI Radeon R9 280 x2 Crossfire
ASUS Sabertooth 990FX
Corsair Vengeance 1866MHz 2x8GB
EVGA Supernova 850 G2
Cooler Master HAF 932