This would be a GREAT experiment to see how core storage handles the scratch file. I'm not familiar enough with PS to know how the file is created, named, over-written, etc. But it would be interesting to see how FD manages the scratch file(s) if you install PS on a non-partitioned FD arrangement. If they are accessed enough, they could/should get moved to the SSD portion of the array. If they get erased each time you close the app, it may have to "re-learn" over time; however, if the same file it written to time and time again, it may get moved.
Would you consider running this experiment? You can install everything on FD with no partition, run it for a while & record your results, and then do the same thing with a partitioned FD arrangement. If you either have iStat Menus, or download and install it for the 2 week free trial, you can see the independent I/O of each drive within the FD array. When I launch apps, I can look at the disk activity history to see where that app, or portions of that app, reside - SSD, HDD or both.
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Agree. And they may with the next iteration. I'm sure you have been following Apple hardware for quite a while, as have I, and you surely see their pattern in continual incremental improvement over time. It's what keeps us wanting to buy "next year's model" when "this year's model" would still be fine for 4+ years down the road! They never offer up the best solution initially - they eek it out over a period of time.