I had a 15" mid-2012 MBP until a couple of months ago. It was absolutely horrible playing StarCraft II with it. When using the dedicated graphics card, it should have been able to run the game on High settings. In practice, however, it could barely manage Medium without lagging horribly.
This was a fully upgraded, top-of-the-line machine with 8GB of RAM. Ultimately, I got rid of it and went with a 12" MBP. Believe it or not, even though this machine only has the integrate Intel HD 3000 card, it play SC II much better than the 15" with the dedicated ATI.
So if you ask me, I definitely think there's an issue with that card or Apple's drivers for it.