I fly the K-4 a lot.  I have not notice any difference in cannon lethality.  You still have to hit the target, which is not always easy with the 30mm.  
I know the rate of fire is high for a 30mm cannon (The Germans traded rate of fire for muzzle velocity), but I believe it is still lower than a Hispano and a lot lower than 50cal.  It is possible for a maneuvering target to fly through the bullet stream without receiving a cannon hit.  Snap-shots are not easy in a K-4.  The rate of fire means you may not score a hit where you may have scored a couple of hits with 2 or 4 hispanos.
On a different note, the video of the 30mm hit on the Spitfire was cool.  Yep, it works like that in the game.  One hit, Spit looses wing.