Light efficiency and watts are different things. This is why, for example, a metal halide, a compact fluorescent, and an incandescent bulb which are the same wattage can give off different amounts of light and heat. Some types of bulbs are better than others at turning watts into useable light.
But in any event, the "cost" per watt is the same. And as a practical matter, flourescents aren't "better" at turning watts into light than MH (both are far better than incandescents though). The primary difference is that a MH is a point source of heat while fluorescents disperse heat along their length. The actual amount of heat is very similar (if you set up an experiment where you did not let the heat escape from an enclosed area, you would find that a similar amount of heat is put off by fluorescents as MHs, assuming the same wattage).
And it is not necessarily true that replacing your MHs will be more expensive than replacing fluorescents. It may be much cheaper to replace two 250W MH bulbs over a given period of time than changing out an 8-bulb fluorescent fixture over another given period of time (which obviously depends on the type/brand of bulbs and how long their respective useable life is).
This is a long way of saying that depending on your setup, it could very well be cheaper to buy and maintain a MH-based lighting system.
(And I'm a sucker for the shimmer.)
