I've noticed expanding diffusers that convert 14.5mm joints to fit 18.8mm bowls. Do these improve the airflow and delivery of the bong or help in any other way, or are they purely for people looking to use their 18.8mm bowls they have sitting around? I'm thinking of buying a new diffuser and bowlpiece, and if these expanding ones do have a positive effect on 14.5mm sized bongs I'd get one and an 18.8mm bowl. If there's no point, though, I'd consider other options. Thanks!
Airflow is determined by the tightest spot of airflow, so those 18.8 will not decrease drag. You need those because some of the best ashcatcheres are made for 18.8 joints (atleast the one you see online), and of course to convert for other bowls and the like.