can't speak to the hydro, the setup isn't difficult, but I could see where it would be less forgiving. There's also a cost involved in starting it up.
You can certainly flower using the t5's, but you already recognize the issue (lack of penetration). I have seen some rather impressive all t5 grows. They all had at least one 8 tube 4 foot fixture within 3" of the top of the plants. Your goal is to grow short bushed out plants. lst, or even scrog would probably work well. I'd be looking to grow plants that were 1 foot, or at most 2 feet, tall, but bushy as hell with a lot of flower spots.
Not having ventilation is really not good though. If you aren't injecting co2 your plants are probably bottlenecked by co2 and you might find that increased light has minimal effect on growth. Venting is not just for removing heat. Just as important (more so imo) is ensuring a fresh supply of co2 in the day, and oxygen at night. In my mind the heat removal is just a nice side effect of venting, primarily I am trying to replace the closet air with fresh a few times a minute.
The heat comparison of t5's to HID isn't quite as straightforward as it seems. First off, watt for watt, flo's and HID put out the same amount of heat. There is so much misinformation on this out there, but don't believe it, the total heat is all about the watts. Now, to make it less straightforward, t5's run at a PF (power factor) of around 50% (according to google). My germination light is 62 watts (2x30" t5) but runs at a PF of .63, so it consumes around 41 watts on average (I haven't hooked up the meter to my 8 tube t5 yet). Basically this means that between a 62w hid and my 62w t5 (pulling 41w), mine is producing 37% less heat, however, if I truly matched the draws so I had an HID and a T5 pulling the same wattage (PF of 1.0 for both) they would produce the same heat. The other thing to take into account is that HID's are a point source, so all the heat is produced in a very small area within the bulb and radiates out. T5's generate heat along the entire surface area of each tube, and this gets multiplied out by the number of tubes, as well as radiates through a much larger hood structure. This all means that watt for watt they are less likely to burn your plants, and have less variation in temperature as you move horizontally, so the heat can be much more manageable. However, watt for watt, with no ventilation, they will still heat up a closet fairly quickly.