ive used that same sized space with a 600W and without a good inline fan cooling the hood, things got too hot. without seeing, or knowing, your setup, id say the 2 fans youve currently got goin wont be enough....but we need more info....
A 6" inline vortex with a speed (router) controller fixed my heat problems with ample satisfaction. very happy with the performance and quality of the vortex fans.
but there are alot of variables to take into account...
-brand of tent? some are easier to cool than others (more flaps, reflective materiale, etc)
-whats the ambient temp of the room the tents in?? (is it in a closet? an open room?
-intake air from outdoors, or inside?
-whats the ventialtion setup? venting to room? outside?
these are just a few parameters that need to be addressed to figure out your best plan of attack. we could tell you to go out and buy a 1K w/ 10" flanges and get a 10" vortex, but, that might be WAY overkill; but, apparently, those two fans youve got, probably wont cool the tent properly. probably.
600s put out a decent amt of heat for the wattage. what types/size of fans are you using at the moment?
give us some more details.
while some strains are tolerant to warmer temps, i dont always run the same strain, so, id rather control the enviornment so anything can grow in it, rather than hope the strain im runnin likes heat. dig?
my advice would be:
1. give us a detailed description of your current setup and ventilation design. pictures help immensely.
2. pondering the replies and opinions you get here, decide what your BEST (and cheapest) plan of attack would be to cool that tent, CONSTANTLY and CONSISTIENTLY, so you can run anything, at any time, in it. fine tweaking a tent for every grow will get tedious, trust me. and we want this to be fun, not a lesson in patience.
and, generally speaking, yes, 600W lights will garner more, tighter buds, than a 400W light, but, likes stated above, its only one of a few parameters that translate into larger yields.
-cap