The majority of skimmers are overrated for the size of tank they can handle, but IMO it's no more so than any fw filter. Any fw filter that's rated for up to 30g has struggled to adequately filter a 30g tank IME, but does very well on a 20g. A skimmer rating is in many ways the same. Once the tank size a skimmer's on gets close to that "up to" number of gallons, it doesn't handle the load quite as well. Most of us don't help that much, as we're tempted to stock a bit more than just modestly and probably overfeed at least ocassionally.
Some hobbyists claim we should use a skimmer rated for twice the volume as our tank, as skimmers are overrated to that large of an extent. I'm not sure how valid or important that is, considering there's a sizeable crowd that runs skimmerless. Most skimmerless tanks are, however, stocked lightly.
Many skimmers can be modded (some extensively) to improve efficiency, while others will do more just by upgrading to the next size pump or changing to an equivalent pump with a needlewheel impellar (instead of just a standard impellar).
I'm not sure if that really answers your questions or just causes more, but I thought I'd toss my thoughts out there since there wasn't much of a response prior to now.