For what its worth, all of the problems relating to the Ki-84 that I've been able to locate have stemmed from improper heat-treating of landing gear components, and mechanical failures of the Ha-40 and 140 (Koh and Tei) engine as a result of poor quality control.
Despite a stack of books on Japanese aircraft nearly 4 feet high, I've never been able to locate a source or reason for the Hayate shedding parts in a dive. That's not to say it doesn't exist (the reference) or that it didn't exist (the problem), but I haven't been able to locate it.
As to the Ki-100 - it was a superior aircraft to the Ki-61-Tei in all but top speed. The reduction of weight in going from the Ha-140 to the Ha-112 improved performance dramatically, making the Ki-100 a formidable fighter for its time. Reduction in overall weight equates to a reduction in wing loading, which improves performance.
Simply comparing it to the Ki-61 and saying 'we already have that' isn't giving the conversion its due. There were countless aircraft that went through this process and the results were superlative aircraft to the originals.
Fw-190A to Fw-190D-9 to Ta-152
Merlin Spitfire to Griffon Spitfire
F4U-1 series to F2G series
P-47D to XP-72
La-5 to La-7, then the subsequent La-9 and La-11 (although new designs and not direct descendants of the 5/7 series)
*Granted, a few of these aircraft never entered production or fell victim to jet designs, but it validates my point*
If it combines the performance of several aircraft into a single package, and creates yet another adversary that we have to think about before we engage, then why not? In terms of 'converting' the Ki-61, most of the 3-D model is already complete, weapons and ballistics data is already there, and engine data is readily available.
If it concerns us that having another uber-fighter (which this wasn't), then perk it along the lines the Spit XIV.
Flame away....
