That was snarky
OP asked about a handheld macro setup...Yes, but this is canonrumors ;-)
comment was the snarky one, sorry!
just showing my Kirk love.
Well, that's ok then...and I didn't even respond with a plug for the RRS rail.
Allow me to remedy that.
If you'll only use non-collared lenses or collared lenses with non-gripped bodies, the Kirk rail is fine (you can rotate the clamp with a hex key for use with a collared lens). But the RRS solution for collared lenses includes a raised platform, so a gripped/1-series body on a collared lens will clear the rail.
I suppose the only catch may be that different parts of the 5DIII AF system are used because the 180mm is f/3.5 and the 100mm is f/2.8
Good point. Obviously, on a tripod one would usually manually focus with 10x Live View anyway. But the macro lenses take a hit on AF performance spec - despite being an f/2.8 lens, the 100L IS Macro is in Group C, meaning no f/2.8 high-precision cross points are activated, just the f/5.6 crosses in the middle and the f/4 crosses at the sides (and all of the f/5.6 lines, too). The 180L Macro is actually in Group G, meaning only 33 of the 61 AF points are active at all, and for cross points just the 15 f/5.6 ones in the middle.
One other thing to note about the 100L vs the 180L is that the 180mm lens takes TCs, meaning instead of 1:1 you can get to 1.4:1 or even 2:1. With the 1.4x TC, it will still AF in Group G. With the f/8 AF update, I'm not sure if the 180L + 2X will AF using the center point or not. While it's physically possible to use a Kenko 1.4x TC on the 100L, there are issues with doing so on the 5DIII - it you have AFMA enabled on the camera, the Kenko 1.4x + 100L reportedly causes the 5DIII to lock up (meaning battery removal to reset).