Problem is, asking for the 30mm on the Bf109G-6 we have in AH is like asking for +16lbs boost and 120 rounds per cannon on the Spitfire Mk V. You can find lots of documents supporting it, but the aircraft in AH are supposed to represent earlier aircraft in their series than the 30mm armed Bf109G-6 and +16lbs boost Spitfire Vs.
I don't think boost levels/limitations can be compared with loadout options that are selectable from the hangar and can now be disabled by the CMs for special events. G-6 was introduced in the beginning of 1943 and first 30mm equipped versions came off the line in the summer of '43. G-6 faces '44 vintage bombers in the MidWar arena.
I would love to see the Bf109G-6/AS added as an additional aircraft though. It is badly needed for scenarios involving combat with high altitude American bomber streams prior to the introduction of the Bf109K-4.
G-6/AS had a relatively short production run (for 109) of 686 aircraft. So I don't really see it as a hole plugger per se. Wouldn't mind one either though
at some point. Finns had two of them.
To the best of my knowledge, the Bf109K-4 was never built with the 20mm cannon in the nose. That was specifically why AH used to have a "Bf109G-10" that performed exactly like a Bf109K-4, but had the 20mm option.
The fact that part of the K-4 production had MG151/20 in place of the MK108 has been mentioned in Prien & Rodeike's Messerschmitt Bf109 F, G, and K: An Illustrated Study.
MK108 was the preferred weapon for the K-4 but because of Rheinmetall Borsig's difficulties in producing enough MK108s, electrical wiring was installed for both cannons so that when MK108 was unavailable, MG151/20 could be installed instead. I haven't seen any specific figures on how many of the K-4 actually had the MG151/20...and it is quite probable that such numbers don't exist or never will be found.
This isn't so important in the MAs (but in the MAs you can fly K4 if you want), but it is very important in scenarios and special events and the AvA.
There's feature currently in the game which enables CMs to disable loadouts for special events.