Actually, why don’t we look at other single engine fighters with a big gun like the 9K?
USA: P-63A-10, quite a good plane for ground battles. Only one BR step below the Yak-9K, yet its gun is so much worse - 37mm of pen at point blank and no HE filler.
Of course you get bombs, but you can’t really fight other planes with them; the Yak-9K doesn’t have to jettison its 45mm gun to do so.
Germany: closest is the Fw 190 F-8 at 5.0. You get two horribly inaccurate gunpods with low ammo count (35rpg), no APHE, and mounted way far out under the wings so convergence becomes an issue. HVAP has terrible postpen. Flight performance is worse than the 9K in all accounts except top speed (when clean, with these HUGE draggy gunpods it might be slower), and acceleration is worse.
Britain: Tempest V Vickers P. Initially looks really good, but it handles like a boat and still uses solid shot, which at this caliber, with unimpressive fire rate and mounted far out in the wings is not great. There’s a reason even at 3.7 these are very rare.
Japan: ZERO true counterparts. No really, they never put a gun above 30mm (that has been added to WT) in a single engine plane. There’s plenty in twin engines but 99% of them suck anyway.
The closest you get is the premium J2M5, with a worse flightmodel than the tech tree one, that gets just two 30mm cannons - of course mounted far out in the wings - and 84rds split across them. 38mm of penetration is only good for light vehicles most of the time, though if you’re willing to spend half your ammo on one target you can sometimes sneak a shot through, say, a Skink’s armor and that will cripple them.
Against most tanks you will have to hit engine decks or just ignore them, and of course shooting planes is ill-advised now that you’re shooting APHE that won’t fuse on aircraft skin and requires you to hit something important. That is usually on the center of the plane. With your wing-mounted guns.
China: even less than Japan. Closest is the… P-38L, I guess? With its nose-mounted 20mm cannon. It’s not even incendiary AP…
Italy: Yak-9P can mount the N-37, same as russia’s Yak-9P. Better engine than the 9K obviously but the gun is quite a bit worse for ground attack, and its 5.0
I guess the EVENT VEHICLE 109 F-4 is also an option, but like others before - low accuracy gunpods, mounted far out in the wings. Ammo count is good (135rpg) but the 15mm cermet rounds deal very little damage to both planes and tanks.
France: P-63C-5 (identical to P-63A-10 above, slightly different engine), OR Hispano 20mms (identical to P-38L’s), OR a Yak-9T with a similar NS-37 to the italian pick but with much more penetration - still SOLID SHOT and unlike the N-37 this isn’t incendiary.
Sweden: 20mm cannons only, identical pen to Hispanos and likely just license-produced copies.
Israel: they don’t have a ground tree at a relevant BR.
As we can see, the Yak-9K is completely isolated and far superior to the other single engine fighter, gun CAS aircraft. Even those with “a lot” of bombs (usually up to 4 bombs big enough to be worth using, on very few fighters) are far more limited in kill potential than 29 rounds of 45mm; and of course, carrying those bombs makes you almost a free kill to enemy fighters that actually know how to do pilot stuff.
