I have not used CFLs as plant lights, but have used fluorescent tubes for this and for starting seeds. Any additional foot candles of light from any source are appreciated by a plant, but there is spectrum and intensity and the actual coverage area (not sure the correct word there) to consider. Some bulbs are very focused, some more wide-ranging in the area they illuminate (obvious example of this is the fluorescent tube vs. the basic bulb, and a spot vs. a flood). So I think you’d have to do some homework on the output and the range of coverage and so on. Also on the color of the light (i.e., the spectrum). Some are cooler, some warmer, and when I have grown with fluorescent tubes I have always plugged in one of each to the metal "hood" to hope for balance. I have also kept the tubes quite close to the plants I’m trying to help. Your east window will not be enough light for most herbs, you are correct. Which ones are you hoping to grow specifically? They also like slightly different temperatures in winter in some cases.