I'm not sure how it is with the English ones, or the German WGr21s, but each American HVAR rocket is equivalent to 250lbs of bombs. 10 rockets knock out a VH at airfields, and 8 rockets knock out the VH at vehicle fields.
If "people have dumped over 20 rockets to destroy a vehicle hanger to no avail but a spity can knock it down in 2 passes with it's 20mm cannons", that means probably about 8~9 of those 20 rockets hit, and the VH was very weak, and the Spitfire comes in and filled the required amount for destruction with strafing.