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I'm a long way from any kind of engineer, but I could give you a physicist viewpoint.
First, watts refers to the power the bulb draws and has only a loose relationship to light output.
One important thing is the lumens (candlepower, etc) which is a direct measure of light output. However, for this to mean anything, you also have to know the spectral distribution of the light, i.e., if it was all in the ultraviolet it wouldn't be of much use to human eyes. Keep in mind almost no light source has an even distribution of color from violet to red. Color distribution is effected by the filament, the gas inside, coatings on the bulb, etc.
Also, since headlamps are most critically needed at night (duh!), one has to consider how the eye shifts in spectral response between the rods and cones we use to see the light reflected back to us. As a side note, consider the various driving conditions (along the lines of why the sky is blue), clear air, fog, rain, dust, etc., and their effect on transmission of the light out to the object we want to see, and the path back to the eye.
Finally, something should be said about light distribution on the road. On a stock SVX there's not much we can do about the headlamp design, but I have heard of people putting high beams in place of the low (just trim some tabs), thinking that the reflector at the tip of a low beam inhibits light output. I have no first hand experience with this, and must expect the designers of the headlamp took this output distribution into consideration. A related item is what effect your headlamps have on another driver's vision - this is partly aiming, and partly how the rods and cones respond after being directly illuminated by oncoming headlamps.
I'm sure there's more, but at least these things would have to be considered by a lamp designer.
Help or hurt?
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