i dunno, as someone who used to break wooden boards and a brick or two while taking tae kwon do, the reason i used my open palm was NOT because it was more effective then my fist, it's because i didn't want to break my knuckles on a brick. There were some black belts and double strike black belts that could break bricks with their hands but after the demonstration they were always in the back icing and putting on herbal medicinal oils on their fist, they obviously felt it.
I was making fun of the guy because saying a well placed palm strike could kill a man isn't saying anything. So what? You name a part of the human body that has a hard bone near the surface of the skin and i can probably find someone who has died from being hit by it.
Technically, a punch is just more effective then a open palm especially given the padded gloves MMA fighters wear.