Steve, the cam you have in that engine is optimized for fairly high rpm, so I would expect vacuum to increase significantly as you rev it up. Check out the numbers Connor Davis got on his EFI 350 in
this post Although no-load readings don't mean a whole lot, notice that he got the highest readings in the 2000-2500 rpm range. I suspect the cam in that engine is probably set up for max torque at fairly low rpm, and I would expect to see the highest no-load vacuum at the rpm where the engine makes maximum torque. Manifold vacuum is a direct indicator of pressure in the cylinder when the engine is under load, so the absolute level of vacuum is meaningful on any engine. A "manifold pressure" (vacuum) gage is one of the primary instruments on all piston engine aircraft for that reason. The cam characteristics will simply determine the rpm at which you'll see the the highest reading.