I'm thinking about doing the same thing. Getting rid of my 3 250 watt de pendants and two 72 inch VHOs and go all T5s. Just not sure how many I'd need to make it work since I'd have to go with the 36 inch bulbs side by side to get max coverage. So I'm guessing between 12 and 16 I could fit into the canopy system I have, for a total of 6 to 8 rows.
Let me know how yours works out.