6x20mms...If I am at 25k (which could burn the drop tank getting there) and the buffs are 17-10k, I could get three good, clean passes. That would mean I have the E to blow through the escorts, pull up, nose over and get two more passes.
After that, maybe even before that, it is a race to the deck and ack-cover with limited ability to fight.
My guess is this was learned the hardway and why only a few examples were used.