Hi Tarquin,
One 'caveat' about the dive test: it only provides an approximate reference point. In other words, you may achieve a positive result for the test, but not have the pipe length optimized.
I am sure you are aware of the new YS 'classic' 60. During testing of the prototypes, the US importer (Rich Verano) was unable to replicate the factory rpm figures (14,000rpm on an 11x9 apc prop). Rich was using the same set-up he had used for his YS .61AR in his Escape and could only achieve about 13,200rpm on the prototype which ran great otherwise. The factory sent their set-up; pre-tuned header and pipe and, voila, Rich could now get 14,000 rpm and the engine ran great in the air. The factory set-up had the pipe length about 1" shorter than Rich's set-up.
I love the OS 'VF' series of engines and still have several in running condition.
Please post some photos of your Subarashii - maybe even the plans!
Regards,
Will