[beatr911]

To test, use or make a flow bench. Tightly seal your most powerful vacuum(s) available to the parts you want to test. Ensure the path leading to and from the part you want to test mimics the vehicle installation as much as possible to consider turbulence in those paths. Measure pressure in and out. Look up flow bench procedures, this is a tried and true means of measuring flow, and there is alot of knowledge and well documented methods and formulas for using them.

You could probably get a gross idea of flow differences by measuring deflection of a piece of card stock over the end of the exhaust from the shop vac, or measure the amperage draw of the vac.

OTOH, the improved stock part is probably pretty good unless you're using higher than normal boost or rpm, certainly outside typical hypermile driving cycles. Making further improvements is probably into the diminishing returns end of the $/benefit or time/benefit scale.