Quite frankly if You believe power to weight ratio it’s underpowered that’s honestly a skill issue
Absolutely not the 9k was a solid plane with a good gun before this the 45 mm had really good overall stats wich Made it a solid plane in my opinión as for your comparison, Ur comparing a 75 mm long anti tank gun mounted on a completely different type of aircraft specifically made for the role of destroying highly armored ground targets meanwhile the yak 9K was equipped with a high caliber cannon for the purpose of destroying enemy bombers its quite frankly an unfair comparison specially with the performance difference between them both