Going up to f4 made a huge difference and was not really an option. DOF is not a concern for this type of shooting, especially since flash will punch out the DOF anyway. My concern with the Tamron is well...it's a Tamron, Ummmm I dont quite think you understand the concept of DOF here flash and DOF are two completely mutually exclussive aspects. ie flash has precisely zero effect on DOF. DOF is a function of the lens and sensor the flash simply provide light for the exposure
the tamron fear is totally understandable however.
the f4 on full frame is essentially going to give you the equivalent as the 2.8 on your crop
on the 5Dmk3 at events even shooting iso 16,000 is fine where as with crop its 1600 maybe 3200 at a push
iso 16000 on the 5dmk3 takes about the same NR and processing as 1600 does on crop IMO
The Tamron uncertainty is understandable, but I have used the lens, and it's quite a nice lens, especially when you throw in the price. AF on it moves a small bit slower than many of the rest of the L lenses, but build, optically, IQ, and VC (Tamron's name for IS) are all very good, on part with the vast majority of L lenses.
That said, using a flash I'd say you hardly need any kind of IS unless you're dragging your shutter way slow (which it looks like you're doing), but the subjects you are hitting with the flash will still be in pretty good exposure due to having a relative ton of light being reflected back to the sensor compared to the surroundings.
So if you're really concerned, go for the Canon 24-70 v2, otherwise I'd rent the Tamron and try it out. You'll find it's quite heavy actually, although I believe the 24-70 v2 is pretty heavy as well.