I only owned one with the early head. I remember noticing more power when I got a 1956 year model, but there were other changes along the way. I figured they all helped in degrees. More than anything else, it seems to me the right camshaft and the right gearing helps with power management/optimization. The best performing inline six I ever owned was the Pontiac OHC. I hoped the Datsun inline six would be good (first version of the Maxima). It ran very smoothly but performance wise never lived up to the advertisements.
