In a Bf109G6/G10 you can definitely feel the difference in handling with MK108 vs a single 20mm, but its nowhere near the difference when mounting gondolas so a 30mm is a better choice for more instantaneus firepower. However overall the single 20mm will give about the same number of kills per sortie as the 30mm, because its easier to hit in a turn and the firepower isnt bad at all because you can, and should, fire the single 20mm and two 13mm at once as the trajectory is nearly identical.