Just because of that ceiling height you'd likely replace with the 6/750s.
When i spoke to a gavita person, they told me 1mm of distance for every watt you run. So i keep my 750s at about 750mm away... running the 1000s that would mean keeping the canopy 3ft away or so. 8ft ceilings, minus a foot because of the form factor of the light/ballast combo, leaves 7ft, 3ft of space until canopy, leaves you with 4ft total to work with, including pots and everything. IT IS POSSIBLE THOUGH, i personally know a guy who rocks a gavita 1000w de in a 5x5 tent with active ventilation. You've just gotta be on top of your shit in there
As mentioned, you should just email gavita, they'll really help you out with those #s