Well...
I can't say what the lower cold limit for starting is, but here's what I've observed with mine:
At 50°F+, the starter "grunts" one or two times (and I assume that each "grunt" is the slowdown that happens when a piston comes up on compression..) before it's running. So, 1/2 to 1 turn of the crank and running.
The other morning, when it was 28°F (coldest it's been since I got it this summer, and y'all up north can quit laughing now..), I went out to start it just to see how it'd do. Didn't plug in the block heater, no grids, nada. I counted about 5 "grunts", so 2.5 turns of the crank, and we were running. So, 28°F doesn't appear to be a problem.
I'll share this trick, too:
If you're starting in the cold, keep cranking for about 10-15 seconds, especially if the thing is just barely cranking over. If you don't start on the first attempt, don't panic. Wait about 2-3 minutes, then try again. I'll bet it cranks MUCH faster on the second go, IF YOU WAIT..
What you're doing is heating the batteries (and pre-heating the engine a bit, as well as putting a little more lube to the bearings..) on the first try. Takes a couple of minutes for it to have a positive effect on battery output. If you don't wait and just try again after only 30-60 seconds, or just keep grinding the thing, it's not going to help.
Of course, optimum is to have a setup that provides enough oomph to do the job at whatever temperatures you operate in.