No, you can't upgrade T8 to T5 (or vice versa) - the ballasts are incompatible and using the wrong lamps will toast the ballast in a short time. There's no need to do so, though. The only real difference is a slightly better efficiency for the T5. Nowhere near the step in efficiency from T12 to T8.
For large plants there is a whole range of products, including HPS (high pressure sodium), MH (metal hydride), and LEDs. You can thank the pot industry for providing the money for developing these commercial products. Perhaps a sordid history, but now we legal gardeners can take advantage. Here's a sample of what is available:
http://www.htgsupply.com/categories/...te-grow-lights