For anybody else coming across this post with similar GPU, I've since downloaded / installed the game via Steam. My machine is running the game perfectly smoothly with the specs below and full display resolution of 2560 x 1440. Only once have I had a slight FPS drop when fighting the "Will-o-the-wisp" enemy (attack visuals), but the next time I fought the same enemy....no issues.
iMac 27"
2560x1440
Intel Quad-Core i5 2.6GHz
ATI Radeon HD 5750
8GB RAM
2TB SATA