Any void space has to be filled with the air that the piston is compressing, so a port larger than necessary will also reduce the maximum potential pressure.

A while back I made a number of screw-in ports for a chap who was experimenting with a Walther rifle. The screw-in ports started out as brass grub screws that would screw into a tapped 'port', and they were all primarily drilled 2.0mm. Then with a taper reamer I opened them out, one by one, from 2.1mm in 0.1mm increments up to 3.0mm which was the largest he wanted. I reamed them back to front, if you follow me, so that the hole in the port diverged marginally, as this ought to give a better flow characteristic than a port that got smaller.

I made the taper reamer from silver steel and the taper was chosen so that I could easily judge the bore size by the depth the reamer had entered.

I think a taper reamer is a better way to go than a twist drill; it would be remarkable if a twist drill was that accurate, as any imperfection in the grind will make it cut oversize.