i have had one of these since 95, it's not the best protein skimmer i have ever used.. but it's not bad for special applications. where you need a mechanical/chemical filter, but have very limited space, and these things make one hell of a racket too. during tank cleaning i got tired of the stock air restrictor falling off every 2mins or getting clogged, and replaced it with better one,like this one:
i did try playing with it's airflow, hooking up a small air pump to the air intake, then fine adjusting the incoming air, however after i got it working.. the bubble production was maybe 15-20% more, and noise it made grew too. in my opinion it's not really worth the effort, the protein output was hardly even effected. stock works fine, as i said it's not a the greatest protein skimmer, so don't expect great results from it, plus protein output is dependent on animal load so increasing the air intake a little will make only a slight change, in any.