I'm pretty sure Simcity only uses a single core, although I can't verify that right now. It's definitely not a heavily multi-threaded game. But the i7 should not hamper performance with or without HT enabled. Are you trying to run it with the integrated video? If so, then I'm sure that is the cause of any performance problem you may have.
Here's the recommended requirements for that game: http://www.simcity.com/en_US/system-requirements