Whoohoo, let's stop right there! How high can you go? Somebody's gotta stop this.
If you put a 1000w in there, that would be 167 w/sqft. Definitely too much.
Right now, with your 400w, you are supplying 67w/sqft. Perfect for getting excellent results. Adding side light will increase your yields nicely.
Go with the T5s, and put them on at least 2 sides.
HTH