Screen mirroring will run at the lowest resolution of either screen, in this case your TV's.
Honestly for best gaming performance (well, as much performance as a crappy intel card can put out anyway), you should use an external mouse and keyboard and use clamshell mode if you plan to use the TV, that way your graphics card doesn't have to run two screens at once.
Clamshell mode dedicates all available video RAM to the single external display, thereby increasing performance. Unfortunately, since OS X Lion you are unable to choose two different resolutions when mirroring a display.
or you just mirror. It only has to drive one display. The hardware that actually duplicates the image for both screens is specialized that won't affect performance (won't take away from the pixel generating hardware and shaders). That is no additional load compared to clamshell. In clamshell mode cooling performance is worse which is why I prefer to keep the display open.
Extended will require some added load but it is not as much as one would think, assuming that the second display content is mostly static. Most of the GPU hardware will still be spent on generating the 3D image of your game, using the shaders and rops.
Just try it and benchmark. The biggest problem will be the weak GPU and the minimum framerate because the CPU has to go slow when the GPU is in use. Turn off or down lighting and shadow effects they punish the weak cpu. Use as low a resolution you can work with. Shader can be higher you have plenty power there. AA and high resolution are a problem for the low memory bandwidth. Textures maybe high but not extra or ultra.