Tom, a Dust Collector is low velocity and high volume, a shop vac is just the opposite low volume high velocity A dust collector does not increase velocity when it is choked down, it has the same velocity and less volume. So if you say choke a 4" DC down to 2 1/2" you get the same as a 2 1/2" @ the same velocity as before but nearly 1/2 the volume as before.
You can tell by the sound of the DC. Leave the 4" wide open , it is loud making lots of noise, cover the end completely it is half as loud, because the motor and fan are not working hard moving air. Open your fingers and it is half again as loud,the motor and fan are working, but not full rate yet.
You can do the same with an amp meter on the DC line. Start up the DC with all the blast gates closed,(mine reads ) 9.6 a. everything closed, and 11.8 a. everything wide 0pen, which indicates it is working harder moving more air open and decreases when closed.