Have some one put it on a flow bench to prove it is crashing at or after .500 lift. Then get them to port it so it don't crash (loose flow) at his lift (.557)... test on car. (There is so much more to this)
That is, making the bowl/valve/chamber hold above .500 lift.
Install it. If the et mph jump, probably the problem. (or you could do like i did and buy a better flowing, more efficient head the flow above .562 lift, my lift)
Then you would have to put to old cam in and see if it make less HP BUT make more them before.
Dont mistake me for a head porter, or someone that knows what he is doing.......I'm not that guy. I have experiment on the j head for 2 years(back in 2012) and never installed it on anything. so i really don't know.