Even if CS22 only requires approximately 1.5 m/s (~300 ft/mn) at sea level ISA, this is really not comfortable.
I get bearly 250 ft/mn when taking off from high altitude airport (5700ft) with the Ventus 2 CM and it is OK but not a pleasant experience! Especially in mountainous environment.

Anything below 400 ft/mn at sea level will result in uncomfortable performance during hot days and/or high altitude.

Concerning electric performance, the motor itself does not lapses with altitude. Therefore, at given rpm, the thrust loss is pretty much proportional to the effect of air density variation on the prop.

The rule of thumb you propose is very rough and would only approximate sea level static.

For climb performance you really need to model the prop and get an estimate of thrust at typical climb speed.
Because FES and GP have relatively small diameter props, they are highly loaded and the GP has an efficiency advantage with the 3 blades.
If you run the analysis, you find the GP15 should have a sea level ISA climb rate of ~500 ft/mn at 470 kg, which is pretty good since you are taking off at 60 kg/m2!