I haven't played these two, but on my 15" rMBP with GT 750M (the same GPU as the 21.5" iMac), I can play BF4 at 1680x1050, mix of high and ultra, FXAA and 16xAF and get 47-50 fps on average, with the lowest being 35 fps and easily shooting past 60 fps in quite a number of situations.
Although the iMac's GT750M is only 1GB (my rMBP has 2GB), it should perform identically because the 750M in both the iMac 21.5" and the 15" rMBP are GDDR5.
I don't know about those games, but my iMac with the GT 750m get's around 70fps on Ultra for Dirt 2 in OSX. I'd say it should run those games just fine.
While WoW and SC2 both work pretty nicely on OS X, D3 RoS is the real mess right now. Performance is horrible, Blizz even can't fix some really annoying bugs for two months now.