I just finished up a rework on a mandolin originally built by me and went to a 7 1/4" radius (cylindrical). It was requested by the owner. At first I was a bit concerned, especially when I was feeling it without strings on. But after strings were on, it's noticeably rounder than a 12" radius, but compared to a 9 1/2" which was on there before, nothing dramatic. It plays very nicely.
One thing that one must realize is that with a cylindrical board that the outside edges get thinner as you go up the neck. This is just due to the physics involved. In fact I would guess, although I haven't actually drawn it out, that various conical layouts could be had of any starting radius and you could determine the radius at any other spot on the board by maintaining the same thickness on the outside edge. Thus your action is same from the nut all the way to the end of the board.
Oh, and one more thought: I prefer to press my frets in with a caul before the fingerboard is glued on. That would be very hard to do with a conical board. You'd need a different caul for each fret.