Update: I've been using an Apple Japanese laptop keyboard for 7 years now, and it continues to be great. I never quite got around to more ambitious layout ideas, but
something like this is still a dramatic improvement over the ordinary US-ANSI layout for me:

I could probably figure out something better to do with the number row than duplicating two of the numbers, but this works for me.
The delete key using the right thumb continues to be excellent, and spacing the hands 2 keys further apart is a huge ergonomic advantage, with significantly less wrist strain.
One silly thing is that Apple decided to move the whole number row .25u to the left on their recent Japanese laptop keyboards, I guess to make the 1u delete key 1.25u instead. So now the number row doesn't quite line up with other keyboards, which is slightly annoying if you need to switch back and forth to a standard keyboard. They also, for better or worse, bailed on the touch bar, so now there is a full row of F keys for people who want extra keys up top.
One final silly thing: Mac OS now uses the name "U.S." to mean "the Latin letters printed on the physical Japanese keyboard", whereas it used to mean "more or less approximate US-ANSI layout". So now if you want US-ANSI layout you need to pick "Australian" as the input source. The standard Japanese Latin layout is in my opinion quite horrible for anyone used to a US-ANSI keyboard (e.g. the apostrophe on shifted 7 is ridiculous), so this naming scheme is mystifying.
For reference, the standard "U.S." (i.e. Japanese Latin) layout
is like this:
The "Australian" layout (on Japanese physical keyboard)
is like this:

And the physical US-ANSI Apple laptop keyboard
is like this:
