While that’s true, AFAIK the source for the Ki-43-III Otsu having a 1500hp engine to begin with is Francillion’s book, which is full of errors and is widely regarded as a pretty bad source.
That book itself doesn’t actually claim 1500hp either, it claims it used the Ha-112 but not its power output:
Which would line up perfectly with the Ha-115-II (that it now has in-game) that added water injection to greatly extend WEP time when compared to earlier Ki-43s.
I believe that’s just a simple typo and everything except the engine name itself says that’s a Ha-115, not the more powerful 112.