I'll jump right on back in...
Wicked is right... there is no need to worry about cubic feet (ie. LxWxH) unless you are looking at a vent for your cabinet. When measuring for lumens just worry about square feet (ie. LxW).
So, you have 10800 lumens (2700x4). You have 2 sqft. That is 5400 lm/sqft. MJ plants can't use much more than 6000-7000 w/o C02 supplementation so you're doing great at 5400.
End of story.
Glad you to see got into nuclear chemistry and not geometry.
(I had to say it... you set yourself up... just kidding though)
I hope this helped,
Born420