You need the correct color temp of blue & red LEDs for plants. LEDs dont produce a wide spectrum of light like most lamp types, so there's no wasted light energy. Most lamps (T5 included) produce a broad spectrum, then the unwanted colors are filtered out to produce the desired color output of the lamp (6,800K; 10,000K; 18,000K; etc.).
To answer your other question, the total light may or may not be equal to a T5, but it is all focused in one direction. A flourescent gives off 360 degrees of light & uses a reflector to focus the wasted half of the output back down where you want it. Reflectors usually mean lost light, but a much wider light beam. Check out Neoprodigy's DIY LED build to see the LED's "spotlight effect" I'm talking about.
