Where are you getting this water injection from? I have 4 book on the Hayate and NONE of them mention water injection. Well one of them might because it's written in Japanese. Ok about the 388mph that everyone is throwing around as the official number for top speed here is a little clarification on that.
From Rene Francillon's book "Japanese Aircraft of the Pacific War"
ISBN 0-87021-313-X
page 231
During flight trials the Ki-84 reached a top speed of 624 km/h (388 mph), climbed to 5,000 m (16,405 ft) in 6 min 26 sec and reached a service ceiling of 12,400 m (40,680 ft).
Keep in mind this data is from the first service trial aircraft (prototypes). Various changes were made to the aircraft before production aircraft started rolling off the line. The prototype aircraft had 1,800 hp Nakajima [Ha-45] 11 engines with a single exhaust port on each side. While production aircraft had a 1,900 hp engine (11, or 12) with a more efficient multi-port exhaust outlet. These engines were rated for 1,800 hp and 1,825 hp for take off. Later produciton aircraft had [Ha-45] 21 engines rated at 1,990 for take off.
As to the questions about octane ratings the higher octane rated fuel allowed the test aircraft to operate with MUCH higher blower settings and thus higher horsepowers. I've seen pictures of at least 2 of the test aircraft and one was the natural metal scheme shown above, but later this was painted olive drab. The other aircraft was olive drab. There is no mention if the US tests were performed on a natural metal finish aircraft or a painted aircraft.
I'm not saying that the water injection never existed on this aircraft, but none of my references mention it. Vermillion would probably have more info on this subject.
I'd like to see the Frank as well, but the fuel quality question is a bugger for sure. If you're going to put fuel constraints on the Frank then you are going to have to put fuel constraints on all late war luftwaffe aircraft as well. Not to mention the fact that you would almost to be forced into taking into consideration the quality control or lack there of for materials and workmanship for the axis aircraft. So then we would need to see random equipment failures and stuff like that. It would be kind of hypocritical to say lets only model poor quality control for fuel but not for anything else.
my 2 cents,
Spritle