Well, I don't really have a set standard at this point. I'm still learning how to use them right, myself. First round with them and all seed plants. They've been everything from 22" to where they are now at about 11" and that's just because the plants are still stretching. It seems around 14" though is about right.
I've not seen any negative impact on the plants at these distances, although, the 22" caused way too much stretch between internodes. At least in my setup anything beyond 18" would be to far.
Light penetration changes as the lights get moved higher as well, so as the lights go up, the canopy gets a bit thinned more. I really only see about 14-16" max depth of true canopy where I expect to have A+ grade flowers.
I see a tendency for plants to toss nut sacks in low light so keeping the underside of the canopy cleaned and stripped seems to be extra important with these, compared to a 1k, which I'm used to.
I'm not expecting 1k numbers, that'd be ridiculous. However, I'm seeing what I'd consider far superior performance to a 400w HPS system - even at this point in the first round of flower. With 2 x 315 I fully expect I'll be able to hit 1.5# with them easily enough. With the right strain, perhaps 2 is not actually unreasonable. I didn't think I'd make that statement, but I can see the potential if done right.
This is the other side of the room, under the other light. It's at 17" from the closest top, currently. I checked.