Well, dont take my info as the watt's pergallon theory, as a pc light system that uses 40 watts would have far less lumens output than a metalhalide or vho. Also, a 22" system does not necessarily mean it has to be 24 watts. I might say get a retrofit kit, make a hood, or if you already have one, do that, buy a highly adjustable retrofit kit. See, with a retrofit kit, in may cases you can add more bulbs than would otherwise be in a light setup of those dimensions. The more bulbs, logically more light. However, an extra fan or two would be needed to compensate for extra heat. As a suggestion, ask around in the DIY forums, on how to make your own retrofit. A DIY retrofit, would let you set how may bulbs you could have etc. Most of this I learned from my LFS, and the biological needs of lumens from my own facination with science and the anemone expert thomas.
Also as another suggestion, why not go for a HOB metal halide, they are about as expensive as 2 T5's, have more lumens output for a smaller dimension.