Had this argument with someone yesterday. He said a 5x15 ft row of plants with 12 x 5 gals will produce more than 6 10 gals. I explained so long as the whole footprint of the light is being filled out with tops, it doesn't make a difference. The only thing that will change is veg time and how plants are trained. Also 10 gal plants will typically grow larger buds than 5 gals. I tend to get a lot of smalls from 5s unless I heavily prune/lollipop. Not saying 5s make only small buds but plants with a larger root mass will almost always produce larger buds, branches, leaves etc.
I would concur that the number of plants is largely irrelevant. It's the canopy area which dictates potential yield, not plant count. Whether it takes two plants or ten to fill a tent, if the area is the same, the yields will be similar. The only substantive difference, as you mentioned, is veg time.
Conversely, I've noticed zero difference in yield using 1 gal, 2 gal, or 3 gal pots. The only difference is fertigation frequency. That's with a typical ~35 day veg period. It's possible differences would be noticeable with an extended veg period.
