As someone who has actually done it......
It's not "cut and dry" and very much depends on the rifle and even the fit of the pellets in the bore. Transfer port length and diameter (volume) is also a huge factor here.
Drilling the port generally doesn't have a drastic effect on power in most platforms, but it's a very big difference in changing where and how much "pulse" the shot cycle has. Use it to tune out bounce basically. Go too big, and you get slam which is worse and harder to remedy.
Take a gun with a long port, such as a D34, and it has a lot of volume there, so the port effectively acts bigger than it's diameter leads you to believe. There's a lot more "dead airspace" between the powerplant and the pellet than in a short port gun like most underlevers.
On the HW 77/97 they have a fairly short and small transfer port volume by comparison. These guns when enlarged SLIGHTLY will usually pick up a touch of power. They are about 2.9mm stock, and I usually drill them to 3.4-3.6mm. Much over that and they tend to get slammy IMO unless using very tight pellets. This is the platform I've done the most work with, and the only one I can post good information on.
Some guns are designed to basically slam, and not work on peak pressure like most spring guns. Two examples or the FWB300 and the HW55. Both have huge ports, low mass pistons, and aren't big power producers. Take an HW55 with a 4mm port and sleeve it down to 3mm and it will make the same power, but piston bounce will be very pronounced and accuracy goes down the drain. Just not enough swept volume and piston mass. Only other example I could think of.
So again, the classic bull$#it answer I hate to give, but "it depends".