I have not done testing myself, but from what work others have done It seems in an all out race effort the J heads are a little better simply because of the intake runner volume. Factory magnums are limited by the pushrod hump.

For a low bucks daily driver build I would argue the magnum head is better. Look at the exhaust flow numbers in the video OP was able to achieve- over 200 cfm at .500 on the exhaust. That combined with modern closed detonation resistant chamber make them my choice for a street build to have some fun with. More static compression can be tolerated on pump gas with the magnum heads and properly set up quench.

Of course you could get pistons and machine work done on the J's to set them up with quench, but for a lot more money unless you happen to own the machine shop.

Just my two cents. I recognize there is more than one way to skin a cat and more than one way to go about it.