Those GM heads look almost identical to the Mopar heads, I've heard the valve angles are all within 1 degree or so of each other but those builders can do what ever they want with them. Most of the gains with the Hemi I believe came from windage control and opening up the deep skirt block they used initially. Even the Ford engine is nearly identical to the others. There isn't any advantage there except the bodies and the teams R&D. Which is why I don't think the Mustang will ever be competive in PS, the car is just to fat.