X vs J- flow differences?

There is no mystery why the J head floor was cast the way it is.

The X head only ever came with 2.02 intake valves, and the final approach of the floor into the bowl was shaped to meet up with a bowl sized for that 2.02 valve.

The J head comes with both 2.02 valves and 1.88 valves.
The floor of the X head doesn’t extend into the bowl area enough to have a proper transition into a bowl sized for a 1.88 valve.
The J head has material added to the floor to facilitate the use of a 1.88 valve.
The bowl is just machined larger when they use the head with the 2.02 valve, but by adding the material to the floor to work with the 1.88 sized bowl, and then machining it out to the 2.02 size results in a sharper, more abrupt change at the SSR where the casting meets the machined bowl.