i have read that you can actually go to like 12" spacing pretty easily if you are 100% greenhouse without much wind exposure. if you are in the wind, it makes sense to add extra space to insure you dont get abrasions to the fruits from swaying.
i lost a watermelon vine to excessive wind recently. nothing to do with spacing... or toms, or anything really, just wanted to bitch about it i guess.
depending on the cultivars you can actually LOOSE productivity on a per plant basis, but as a whole you almost always get even better productivity.
the most productive greenhouses run rock wool slabs with 3 plants per slab which comes out right around 12 inches on center. imho thats the best, most productive system... but when you run that outdoors, you need to angle off the wines in a w shape so they have a few extra inches towards the top.