I hve never thought abt that but if there is diff onset for G-meters it might be linked with what you want to measure.
For ex if wind gust is what you are seeking then the onset will be very small (less period -> high frequency). On the contrary if you are trying to evaluate pilot action on the plane, what you absolutely want not is to see some histerisys on your nice curves from the interferences of the wind gusts.
That said, I don't think it's a mater of discussion here. We have simply to assume that RR test-engineers knew what they were talking abt. And as they were certainly among the best in the world at the time well....
SO 0.9G wld be 0.9G corrected of any parasite accel ... If you add the fact that there is not atmospheric model in CoD until now (?) then ...
By the way yes it was a disadvantage but pls remind that ALL the Fighters in the world at that time might HAVE HAD THE SAME PROB except those that had Fuel Injection in 1940. Let me guess they were German
If you run for the war earlier that's for sure you'd get some (unfair ?) advantages.
Si vs pacem etc... etc...