This is the speed and climb performance table of H6K4 and K5.
『九七式飛行艇二二型(金星発動機五一型装備)操縦参考書』昭和17年7月 および『現用機性能表』昭和18年5月1日 よりfrom "Pilot's Reference Manual for Type 97 mod.22 Flying boat (with Kinsei 51 engine)" Jul/1942and "Table of Performance of Service Airplanes" 1/M
warbirdperformance.livedoor.blog
I attached the original table. Apparently critical altitude is rounded off.
Also, I think H8K5's "200.5 knots at 5000m" is typo because it's too slow. The way I see it, it should be "205.0 knots".
View attachment 651451
This is the preliminary intelligence report on H8K2.
It is based on the seized official manuals so specifications and performance were so specific and reliable.
These are short essays written by KIKUHARA Shizuo, an aircraft designer of Kawanishi/ShinMaywa.
They are all written in Japanese language.
I hope this helps you.