the game only uses 2 threads and the main thread is clearly maxing the core it is running on. Combined with the other thread it results in a 30-40% cpu usage on a quad core cpu. So you are indeed cpu bottlenecked at your current settings. This also means that you could increase the eyecandy on the game without suffering a fps hit since the gpu isnt really working that hard at all yet.
SC2 cpu and multicore scaling:http://www.techspot.com/review/305-starcraft2-performance/page13.html