In a many vs many you don't change the simple fact that you can hit targets faster, with less target acquisition, and you can get more done with a single nose 20mm.
You don't have to kill in a single ping. With 30mm you're wasting a higher percentage of rounds for every one that hits. With 20mm you can hit up to half your rounds in a short burst. You don't HAVE to kill the guy, just scare him, disable him, smoke his oil, etc.
Many vs many, or 1 v 1, it doesn't change the effectiveness of the gun. It simply requires better piloting skills.
As for the Mk108, do some checking up on the overall Bf109G-6 history. Keep in mind our airframe is a mid 1943 airframe. Also you mention your nation getting G-6s with MW50. That makes them equivelant to G-14s (any G-6 with MW50 is basically a G-14). MW50 kits were pretty rare and scarce in early 1944. It wasnt until the second half of the year they started making common appearances, hence K-4s, G-10s, G-14s, Fw190Ds (early models of which didn't have MW50 because it was in such high demand!) and so forth all appearing in large numbers in the second half of the year. You may have records showing you got some MW50 kits, but for the most part it wasn't common place, nor was the Mk108. Maybe for once your country got the equipment it needed and somebody in the requisitions bureau did the right thing? (hey, it's possible!)