I can tell you I would have bought the Speedmaster heads instead of the money I spent on my 596 heads..
But now I have both and I'm using them both... I've never had them side by side as far as j heads x heads or 596 heads but I'm told by the Internet lol that there isn't much difference.. the Speedmaster heads are more of a Edelbrock copy... Can't really say I got anything out of the seat of the pants either from going from 596 heads with 202 valves to the speedmasters..