Just ballparking, but adding 500g to m2p is adding 50% more weight while adding 500g to p4p is about 35% more weight. Can make assumptions from that I guess
I flew my Phantom 4 with 500g of external batteries almost daily for a year before one of the motor arms finally developed a crack. It can take a licking and keep on clicking! The extra weight actually made it more stable in the wind!