Than why not just go with a standard 4790?
Oh wait, I think I just figured it out (correct me if I'm wrong);
The iMacs airflow is so bad that theres no i7 CPU capable of running at full speed. The 4790 (non-k) at stock speed would hit the threshold the same way as the 4790k at stock, but you can't throttle the 4790 as it doesnt allow you to adjust the clock speeds. SO, because Apple prefers form over function and insists on the design despite the bad airflow, they're FORCED to go with the 4790k, essentially a compromise.
If that's the case; WOW, that's lame.