I started playing this again during the sale and find they have improved immensely since a couple of years ago. I have also come to the conclusion the LA-3 and LA-5 are horribly outmatched by even the earlier war FW-190 A3.
I love the P-38, btw.
The sim has now had multiple major updates and I absolutely love it. As for the Soviet fighters, most of them are very maneuverable but the firepower is lacking. It's not until you get to the late war fighters 1944/1945 that you start getting some fighters with a decent amount of ammo and firepower that approaches that of the German planes.
Yak3 (1944) has the same firepower as Lagg3 (1941).
La7 (1944) has the same firepower as La5 (1942)
Yak9T (1943) has a bigger gun, but can be compared to P-39 (1942)
I think it's more accurate to say that soviets increased their low altitude ability gradually and got the upper hand from 1944 onward, than say it's armament related.