I think you have reached the limit as far as heat is concerned with 800 watts. In my opinion if you add the extra 400 watts, you will need to get more ventilation and extract more heat out of the tent. On the other hand 800 watts is overdoing it in the 16 sq ft you have. If you leave it like that and have a temp of 89-90F on a daily basis I think you would be wasting the extra power for little gain.

keylime