Thats a bit low for flowering a space your size. I'm assuming with the T5 you've not much room for more lights overhead? Perhaps you could drop a few cfls in there or get some more fluro's and mount them on the walls somehow? Other than that you could make a scrog and concentrate your plants budsites directly under the T5.

Here's a link with good info on how many lumens you need

http://boards.cannabis.com/indoor-li...uch-light.html

Have you had a look through the closet/cabinet growing section and grow log sections for T5 grows? You might get some ideas and you'll be able to see what kind of harvest the lights are capable of.

Good luck!