It's not exactly summetime temperatures, but it's livable and I am sure the fungi like it. Meramec Caverns, for example, are 58 degrees F year-round.
The [Spoilers] put off less heat during winter months?
I wouldn't be adverse to magically influenced crops. I'm sure that Toady would make them subject to whatever options he puts on the eventual magic system.
That being said, I recall seeing a documentary on mining and it gets hot down there. Apparently one of the big challenges is not only to keep air circulating but keeping it cool as well. Now, needing to design a fortress around air circulation with shafts and stuff... That would be very dwarfy. Not full air currents and convection, you think magma oozing will kill your fps... Just something when the cave-ins get updated. Maybe path to fresh air or have the path-finder mark areas as it does it's usual routine. Don't know how far down you'd need to go to get uncomfortable and subterranean creatures would likely have a higher tolerance (and a comfort range) but it would justify the 15 levels down thing. (And make invaders loathe to go too deep.)
Gotta remember to check out that farming mod. I was just watching Soap and thinking, "My dwarves need pigs."