My take is you would get essentially identical velocities with identical pressures "IF" the load in the smaller gauge was proportionate to bore diameter. the "Fly in the Ointment" is that smaller gauges are nearly always loaded heavier in the smaller gauge. to be proportionate to bore diameter the loads would be equal to the squares of their diameters. Thus a 16ga shot load would be 82% that of the 12 & a 20 gauge 71%. As many "Low" pressure 1ga loads are in the 7/8 - 1oz range what you are thus looking at to maintain similar pressures is 3/4 oz in the 16 & 5/8 in the 20. Even then with each step down in gauge a higher % of the shot will have bore contact or more wad contact for the same percentage of load.
There were sound & solid reasons why with the older shells the 12 gauge reigned Supreme.
