Then to me that sounds as if there is a non-return valve somewhere where it should not be: "Unchecked" crankcase pressure should not split tanks, checked pressure MIGHT split tanks.

FWIW, it is hard to make any prediction extrapolated to size, but on my smaller glow-to-gas conversions, when breaking in on gas I typically see a 500 RPM gain when comparing "first run from the box" to "fully broken in". But then I am only referencing to engines about half the size of your ST.