...
Just to touch a bit more the 109 subject, i too was thinking about MG FF wing guns on the F, but only on the F-0/1. Keeping those on all the F and G would make the aircraft too heavy although with formidable firepower. Could the MG151/20 be fitted inside the wing (rather than in pods) as well, replacing the MG FF from 1942? One possibility to keep almost identical performance imo would be to have the DB-601E and DB-605A run on C3 fuel though.
Oh - I'd have three cannons on the Bf 109F as minimum. Meaning two in the wings, one in the fuselage; no cowling MGs.
The MG FF and MG FF/M cannons were featherweights at 25 kg, ie. lighter than the .50 BMG AN/M2. It's ammo was also light, 160 g for the Mine shell (140-150 g for the .50 BMG). The MG 151 was already at 42 kg, Hispano at 60 kg; these two more powerful used a heavier ammo, too. The Spitfire with just two cannons & their ammo carried greater weight than it would've been the case for three MG FF, and, once we add 4 remaining .303 Brownings (~44 kg) + ammo the weight goes obviously further up.
The MG 151 without redesign of the wing won't go easily in the thin & small wing of the Bf 109. The Spanish bite the bullet, so their Buchons were able to carry the big & powerful HS 404, the 20 mm cannon from 1951 on.
C3 fuel should be hefplful under 5-6 km, another option might've been the introduction of intercooling, like the Jumo done on their 211F to create the 211J. Of course, the 2-stage supercharged DB 601/605 or Jumo 211 would've been great, but it wasn't meant to be, at least not for service use.
However, they could just do as they did with the FW-190, remove the wing guns to lighten the machine.
Kinda defeats the purpose - have cannons, will kill

However - once 109 and 190 started having the MG 131s installed under cowling, they lost immediately 10 km/h. So, if something must go, trow away the cowling MGs (tiresome, I know). As it was the case with some Fw 190s that have had MK 108 installed in outer wing.
Back to the Regia Aeronautica, have been trying to educate myself more on italian aircraft and engines. So as their development started at the right time to be available for our TL (mid to second half of the 1930s), if we somehow get the 1250HP Fiat A.82, 1350HP Alfa Romeo-135 and 1500HP Piaggio P.XII to enter service in 1939-1940, complementing either developed Asso inlines or just plain DB-601s, then the italian machines would be much more formidable. For instance the Ba.88 which had such a sad story might have been a different beast with two 1250 or 1500 HP engines. Probably even two DB-601s (which they were contemplating in OTL) will transform it.
Not many countries have had 1250-1500 HP engines available in 1939-40, so the RA ight use anything that capable.
The Ba.88 have had problem with construction methods. It employed the caging, as used on Hurricane's fuselage for example, while around that it sported the ribs, longerons and aluminium skin, just like the modern stressed-skin aircraft. Thus it became to heavy. A new engine might not be able to repair that problem?