16k in 7 minutes? That's only 2,200fpm climb, sorry, that's no better than the Ki61. Do the math, 16 / 7 = 2.2 (truncated).
Like I said, the radial engine on the Ki100 was not a performance issue, it was a reliability issue. Less Ki100s were made than Ki61-IIs, and as has been mentioned the Ki61-II was pretty rare. The Ki61-I we have no actually has the light performance of the -I but the top speed and power of the -II, so what does it matter what the Ki100 can do? It won't be lighter. It won't be faster. It won't climb faster (2200fpm) It won't do anything, basically, except be more draggy.
EDIT:Grunherz, you are correct, it has a radial. I copied and pasted, and forgot to change the engine listing (lol) my bad! The other stuff is right though. I don't know where the climb rate my book has came from, the math doesn't add up to that.