Barefeats, I would be interested to see what happens when you compare your i7 680MX results with what AndiS is finding out in his thread seeking benchmarks for the i5 with the 680MX:
https://forums.macrumors.com/threads/1509171/
In your results so far, the i7 is trouncing the iMac 2.9 = 'late 2012' (21.5") iMac 2.9GHz Quad-Core i5, GeForce GT 650M GPU pretty thoroughly.
Based on the arstechnica articles that came up in AndiS's thread, I made the same choice AndiS did: an i5 with the 680MX. I'm expecting that the i5 v i7 will not be a big deal for gaming. The size of the differences between your iMac 2.9 and your iMac 3.4 have made start to worry a bit because they're bigger than I expected.
Cinebench and Geekbench seem particularly suited at highlighting the i7's advantages (which is a good thing), but I am wondering about day to day work (MS Word, Stata) and light gaming (Blizzard games for me, maybe Eve Online).
It's a neat opportunity to isolate which performance differences are coming from the CPU and which are from the GPU.