I'm guessing you could get by with less light than most people are recommending, and different color temperatures, too, although those they recommend might be more like ideal.
I just started growing some arugula indoors (though not hydroponically), using two regular CFLs that weren't advertised as grow lights (probably 22 watt or less, each, and probably 2500k), and the arugula (as well as my other plants around the light seem to be enjoying it, even though they don't grow toward it very fast; there's more to a light than how fast plants grow toward it. I'm guessing they grow toward blue light more than red.
The arugula is still young, and I haven't tried lettuce, yet (although I may try some Merlot lettuce soon); so I'll have to update in a while. I am, however, keeping the lights on longer than others here with their brighter lights (only giving about five hours' darkness or less). They also get some window light.
For flowering/fruiting plants, I would actually recommend using more than one color temperature (like a 2500k one and a 5000-7000k one, if not more than two). However, for lettuce, you probably want mostly the higher color temperatures which should help the leaves grow most.
The color of your lettuce may impact your light requirements. I'm guessing since Merlot lettuce is dark red, it would prefer a higher color temperature even more than regular lettuce, since lower color temperatures have more red, and the lettuce would reflect more red (rather than absorb it), because it is red.
Anyway, I'll probably get more and different lights in February, and update this post, but probably not before then.