I talked with Jair Gavita about this very problem. He told me the 4x8 would do great things with two of the 6/750e inside. That is what I am currently running.
I have a little over two feet from bulb to canopy. I also don't let the girls stretch and water a little every day. I keep the gavita 6/750 dialed down to about 600.
I'd like to try these as soon as possible. I am running Gavita now and do feel I'm getting used to usin them in a low ceiling grow area. How much and where to purchase as soon as known please.