rm41400
i fully understand your question that is why i worded my answer the way i did. to be a little more technical the 12-1 will remove the 'gunk' faster. it may not pull more on undersized systems but would be more efficient at doing it, that is why i went with a 12-2 on my 180g rather then an 8-4.
to oversimplify, your current skimmer is pulling 1 qt of skimmate per week, the 12-1 would pull that same amount out in about 1/2 to 2/3 the time. you'll always have something in the tank to pull out so the larger skimmer will give you better water quality and i don't think you'll ever get into the diminishing return.
kc