Ok, well with what Mr Speedy mentions about the fuelling leaning off, I'd lay a firm bet there's detonation/knock at high rpm/load, and it's that, coupled with the high rpm, that's causing these rod failures.
Likewise, if bottom-end lubrication isn't sufficient enough (hence asking about the bearings condition), that would naturally induce extra heat into the rods, and with the additional/varying heat cycles, that may possibly also weaken the rods.
See what you can find out about the lubrication, and if all is good, and if you can keep the fuelling happy at the top end + don't be too rev-happy, I reckon you'd dispel that 300 lb-ft myth...