So I'm installing some aluminum blast gates from Rockler - they are decent quality "made in Taiwan" generic cast aluminum, appear to be like the Jet and all the other blast gates.
I decide the wall is much thicker than it needs to be and just so happen to have a fresh 2" 80 grit drum on the OSS.
Here are the results. The ID went from 3-33/64 to 3-44/64. Wow, 11/64, I'll have to dig out my fluid mechanics text to see what the difference in pressure drop might be.
At the same time I added a strip of velvet tape inside to help keep the gate from sliding too easily, and also a strip of tape to the outside to prevent the "self cleaning" leak when it's open. The way it will be positioned will more than likely keep it clean.