I'd figure if it were to much restriction it would blow the converter seal, especially on a 700R4/4L60E.
My filter adapters have 1/2" pipe threads, being fed by the factory 3/8" tubing(its what1/4" ID), I dont see that as being a restriction. All I know is my camaro's 700R4 fluid stays clean now.
I think alot of it is myth started by trans shops not wanting to warranty there transmissions looking for an excuse. Anyway to each his own, I'll continue to run mine until I'm proven wrong.