On the Japanese side he talked about how every engine was hand made for that aircraft so easily overhauling them and switching them between airframes was difficult as opposed to the R-2800, which in comparison was plug and play.

this his posited was why Ki-61 serviceability rates were so low, despite being a decent aircraft.

Hm, do you have a page number for that? Just reskimmed the tech sections and couldn't find it at a glance. I'm not an aero-engine expert, but it raises an eyebrow given no other reference work or even Japanese aero-engine work mentions this.