The game only uses one core, that's all it ever did, so there was no performance hit because of that. However, as far as I know the recent patch locked it to the first core.
If it didn't, I'd recommend using one core per game. Also configure your game which is on auto-follow to minimum settings, disable sound (not just move the slider to the left, turn it off completely), and minimize the game window. Depending on how much load is on the system minimizing the window may lead to lost connections - in that case go to your windows control panel and set the system timing to be optimized for background processes.
And in closing: Nice system you got, would love to have something like that.
