I'd say you definitely want TTL for run-n-gun shooting such as convention hallway shots.
Lack of ETTL is one of the biggest failings of my Steampunk lighting rig (essentially a giant heavy wearable dual flash bracket). With practice and considerable awareness of my surroundings, I can often get pretty good results. At Florida Steampunk Exhibition East, where the con was in a single building, and there were light-colored ceilings and walls everywhere, I was getting pretty good results. However, at Dragon*Con where every building and room was different, I bungled many of my walkabout shots with it, and ETTL would have really helped me out.
As for the 1/3 stop of shutter speed for your sync - I wouldn't make that a deciding factor. With an ND filter and/or judicious use of an appropriate aperture, you can block as much ambient as you like and still have plenty of flash power, anywhere except in direct sunlight. you'll definitely have no problems in indoor spaces like convention hallways.
The 5DIII is rated for a 1/200 sync speed, but with my Cactus triggers 1/200 wasn't reliable, I had to slow it down to 1/160.
For my setup posed shots, I took a ton at Dragon*Con with two Yongnuo YN-560s, and had no problem killing as much ambient as I wanted to. I used a 3-stop ND for the daytime outdoor shots (the Soul Calibur set which were in a parking deck at about 2pm, and about half the Once Upon a Time set). Just carry a bunch of spare batteries and change them as necessary. My eneloops never failed me.
I've posted some in the Picture of the Day thread, first post is here but I've posted more (including some explanations and breakdowns) over the next couple pages: