The gpu is mxm but the 6970m is the highest mxm card imacs use. The newer imacs use embedded gpus. Any mxm gpu won't work since it has to be compatible with mac.
Have you took a look at ram usage? I don't think you would be using 16gb on a wii emulator. Also is your ram really running at 1600? Because that is an overclocked speed and as far as I know you can't overclock so it defaults to stock speed 1333.
The i7-2700k is going to be a waste of money. It is only 100MHz faster than the i7-2600k. That is less than a 3% increase in raw performance. Probably at best that will translate into a 1.5% increase in games if those games are CPU dependent. I suppose an emulator is very dependent on the CPU so you may get the full 1.5% increase or maybe slightly more.
Why would an iMac have an i7-2600k? I really doubt they come with motherboards that allows you to overclock.
Try to disable Hyper Threading (HT) in the BIOS. HT generally causes around a 2% drop in performance, but who know? Maybe it causes a bigger drop in emulators.