My main issue with the slow x-sync is not the difference to 1/200s, but that you have to set 1/2ev exposure level increments to get it (or it's 1/160s) - but that *also* means you can only set 1/2ev flash ec steps, which are far too large steps for ettl metering :-(
I didn't succeed yet to force 1/180s on the 6d in 1/3ev more through Magic Lantern because the canon fw forces it to 1/2ev steps... the only way to combine max. x-sync with the smaller steps is in Av mode with a fixed 1/180s shutter speed (in the flash menu).
I'm curious about what you're describing here. I have my 6D set to 1/3-stop increments. Shooting in M with a speedlite, I can adjust my shutter speed and aperture in thirds up to 1/180 s (of course, no HSS involved). So it goes from 1/160-s to 1/180-s, in a 1/6-stop increment, but otherwise it's 1/3 of a stop, as in from 1/125-s to 1/160-s. I use E-TTL, and I can also keep doing 1/3-stop FEC adjustments without a problem.
Am I missing your point here? Basically I don't understand what you mean about being forced to work on 1/2 increments (which I agree it's limiting).
I'm not commenting on the shelf life of the 6D. For my use as a hobbyist, I hope that I'll be replacing the camera before it dies. I'm honestly curious to know what is the limitation you're pointing out.