I recently purchased a EOS RP so also got a couple EF-R adapters for my EF lenses. Does anyone know if the extra space between the lens and body using these adapters offers any macro advantages?
The EF-R adapters are not a substitute for extension tubes. Why didn't you just simply try it and see for yourself?
The adapters serve two purposes. First, the adapters makes an electronic connection between the lens and the camera body. The second purpose is provide the correct flange distance between the lens and the sensor. The EF lenses have a much longer flange distance than RF lenses.
As for trying it myself, I'm not sure of the difference in image size or distance with and w/out the adaptor, particularly for the 100-400 EF lens, so am not sure how to compare since I can't shoot w/out the adapter on this body. My question was mostly curiosity, since a macro extension tube is basically used to increase distince between the lense and sensor.