he needs to stop drilling now
my gut tell me that any more than 3.2 probably won;t be optimal when the correct piston weight is used.
Simply put, if (all other things being equal, and they are not, but with the JSB domes they are about as close as you can get), if the rifle generates more power with lighter pellets, it's means the piston is bouncing too much. Lighter pellets will move faster and reduce the back pressure (this is why the all things being equal caveat is so important - differnt skirt thicknesses, pellet start pressure etc all influence it), reducing the energy lost through piston bounce.
So if you piston is bouncing too much you can
i) up the preload. Will also give more power - but in this case due to cocking effort, he doesn't want to - that's fine.
2) increase the piston weight. Will also give more power.
3) open the port.. unlikely to give any more than a very small increase in power once you get much above 3mm unless the bounce is horrendous, but it may well reduce the felt recoil.
4) increase spring preload (swap for a longer softer spring, so cocking effort is unchanged, but spring force on the piston when uncocked is higher, reducing bounce). Tricky with a ram, which is why I suggested trying a spring
clearly the reverse of all the above applies if more energy is realised with the heavier pellet.
However... oppening the port is the last thing I'd do when I see the piston bounce, because the smaller ports help cushion the final (not the first) forward stroke of the piston, making the gun softer to shoot. So I'd have left it at 3.0mm, done the pellet test, added piston weight, etc. 3.1 or 3.2 may be optimal, but 0.1mm is quite significant on a long TP.