I think you're pretty much right. The concept of the fluorescent light is excited gas... CFL's are simply self-ballasted. The higher watt bulbs are always larger. They are all based on the same ballasting tech...
With those assumptions in the open; I believe you are correct. Each fluorescent bulb higher in wattage, based on the same T8 mini ballast, is simply producing more lumens, not any more "brightness." So you're getting more light, just not any stronger or penetrating.
I've been wondering about this topic myself... Should I get fewer, more powerful CFL's or whatever's most cost-effective? I'm pretty convinced all I need to worry about is the light coverage, because no matter how many CFL's you put in, they wont penetrate any deeper into the foliage... Make sure the highest lumens are where they need to be on the developing plant, and that the plant is not searching far for the light it craves.
I guess the only thing you really need to consider is how effectively your CFLs are dispersing their light to the plants... Try to concentrate lumens where they need to be, because that's all you can do with CFLs. They're low wattage for a reason.
I wonder if there's a theshold... Oh yeah, before anyone zelously attempts to bash my rationale, remember I'm still learning.