4 x 24w is what I use in my own mother cabinet. I can keep 8 plants comfortably under that fixture. I've stretched it out to the occasional 9 or 10 and it was still alright. Yes, you are going to have a 6 inch perimeter around the entire garden that won't be getting the direct light, but branches that hang out into that space will still be fine. You will certainly notice a difference in temperature due to the less wattage. Also the bulbs don't radiate heat as far away as HPS do, so you can keep the plants closer.
Are you gonna flower under T5 also?
No these girls are getting a new home built right now, 21x14 with 10-12 nanolux DE fixtures.