I too have the same macbook. I do connect it to a 22"Screen and have been running DCS A-10 at 1280xsomething. All set to Low/lowest, and get about 18fps in single missions. Just playable. But have to wish for more. I was also in doubt like you on getting an iMac. My budget constrains me to just getting the base 21.5 inch iMac option(1). In my country it cost about $1570.
And with the new model 21.5 inch not having an option to upgrade the RAM by user (no BTO in my country). I find it very limiting.
So I am finally making up my mind to get a PC for about $1300 (Things are about 30% more expensive here) to Run Games and Houdini. My Gaming includes only Simulators. (X-Plane10, DCS A-10, IL-2 1946, LockOn, Falcon4 AF, netKarPro, F1 2012).
What I am eying on is: Intel i7 3770K, Intel DZ77RE-75K, Geforce GTX 660Ti, 16GB RAM
I had wished that with iMac Apple should have added a better GPU with better cooling system and allow user upgrade of RAM as standard. Or even equip the MacMini with a dedicated GPU at least.