If the models have similar assumptions
wrt water vapor feedback, then the model accuracy rests on the accuracy of such assumptions.
WRT water vapor influence, you are probably right that there is a modest net increase in water vapor amplification at higher average temperature.
WRT water vapor amplification, I suspect that the basic (radiative only) amplifying effect of water vapor, which is something less than a factor of 2 over the CO2 - only effect of ~ 1.2 C, IIRC, will be close to the same across a range of average surface temperatures.
Not exact matches
The problem with the cartoon
wrt this paper: It's not clear the veracity of Step 2 (
water percolating from the top).