Spitfires arn't exactly weak - they have much fatter wings which soak more bullets then say a Fw-190, in a turn fight a spitfire exposes those fat wings.
Ki-84 has a pretty rugged airframe, I would put it on par with a P-47 in gamewise, I've seen little as 20-25 50 cal rounds take down both aircrafts and i've seen multiple snapshots in order to bring them down.
Prime example: Shot down a P-47M with less then 25 rounds in a 109k4 (no 30mm) I was lucky he tried to run and i just managed to get a quick burst off.
TWCAngel was in a Ki-84 last night, I dove down and put literally 20x 20mm rounds into him, I took my snapshot and flew off all the rounds hit the rudder area and I expected him to lose a tail or be shot down - I climbed away after the burst and was amazed he was still flying on with only an aileron and rudder missing.
Damage modeling can be tricky, I've been lucky to take a few 20mms in the wings of a spitfire and not lose a wing, I have lost all guns on one side though.