Interesting Ghosth
Personally I use the same convergence for either gun set on the La7.
Ballistically (how they shoot), the ShVak 20mm Cannon and the B20 20mm Cannon are identical. In fact they both use the same ammunition.
The real difference is that physically the B20 is a much lighter gun than the ShVak.
Name Ammunition Rate of Fire Muzzle velocity Weight
ShVAK 20 x 99R ( 96 g) 800 rpm 860 m/s 42 kg
B-20 20 x 99R ( 96 g) 800 rpm 860 m/s 25 kg
So if you look at the weights of the x2 ShVAK's plus ammunition (400 total rounds), and the weight of the x3 B-20's plus ammunition (450 total rounds), you'll see that the 3 gun aircraft is approximately the same weight (or maybe slightly less depending on mounting hardware) as the 2 gun aircraft. In other words, you won't get a performance decrease to have 33% more firepower.
Therefore, in my opinon, I can't see a reason to fly the 2 gun La7, versus the 3 gun La7, unless your going for historical accuracy or the slightly long firing time, but with 33% less Lethality (firepower).