T5 wattage depends on length of bulb. The 80watt bulbs are 72 inches long. 54 watts for 4feet, etc. So really the length of your tank will determine wattage. Some of the icecap ballasts will overdrive t5 and produce slightly higher wattage while shortening the bulb life. Individual reflectors are key to getting the most out of a t5 system, I recommend icecap or TEK reflectors for maximum output (I have TEK on my tank).
Also, if your going for maximum light output you'll want to fit as many t5s on your tank as you can. Manufacturer reflector setups may vary but I don't think you won't have any problem mounting 4 individual reflector bulbs across a 12 inch deep tank (I.E. 55 gallon) or 6 bulbs across an 18 inch deep tank. If you've got a 180gallon or 24inch deep equivalent you could even run 8 across.
T5 bulbs come in a variety of flavors and you'll probably want to mix and match to get what you're going for. They have daylight, 10K, 11K, 12K and actinics. They are the most energy efficient option in general use as they deliver the most PAR per wattage. A 4ft T5 setup can put out the same amount of light as a similar VHO setup that uses twice the watts.
You shouldn't have problems keeping most coral under a nice T5 setup and would even be able to keep SPS if you went all out on your setup.