Remember that the two HMG in the nose were a lot slower firing due to the prop, the Italians estimated it halved the ROF.
The Bf109 F2 with the 15mm was very lightly armed proven by the fact that any pilot with influence had the 15mm replaced by a 20mm . The first F2 captured by the British had already had the gun replaced.
I'm saying, compare a Russian fighter with a slightly dodgy 1,180 hp Klimov engine and a cannon thru the spinner, slightly underpowered and wheezing during climbs, vs. the same fighter with a 1,480 hp Merlin XX and only two heavy machine guns but ... (not sure how to estimate this precisely but I'm going to say...) accelerating and climbing much faster and with a 20 or 30 mph faster top speed. Assuming you are a Soviet pilot and can't have it all, which would you prefer?
For that matter would you rather fight in North Africa or the Russian Front in 1942 with a Bf 109E-4 with two 20mm cannon +2 machine guns or a Bf 109F-2 20 mph faster but only the 15mm cannon and 2 mg?
Anyway, is there some reason why they couldn't figure out how to put a cannon to shoot through the prop spinner in a Merlin the way they did a DB 601?