It depends on how big you let the plant get and also is this the 4 bulb or 8 bulb T5? I've had to downgrade from a 1000 watt to T5 and some cfl's and pulled 2.25 oz's off three plants.
My suggestion would be if your set on using the T5 is to invest in a bunch of CFL's to use with it during flower. OR for the money your gonna spend on a 4ft T5 just buy a 150 hps and then use some cfls for veg. might take abit longer too get a proper sized flowering plant using cfl but at least you'd be able to flower with a 150hps. Just my two cents.