A plausible theory but in reality susano is the embodiment of a users soul, remember if senju's can externally portray their power due to them being yang uchihas being yin internally exert it which explains the high level jutsus they can perform along with susano which is a spirit poured outward due to susano not just being a chakra embodiment it is a spiritual embodiment notice how each susano looks like their caster you can look at kakashi's susano for example who had the same slash mark as kakashi did. Now for the reason madara felt no pain is not because of large chakra reserves but also there's a thing called adaptation that humans have, it allows us to get used to something and adapt to it. Think about it if any of you know anything about certain diseases is that when we're new to it we suffer pains, symptoms, etc from it but gradually we begin to build up an immunity if you will towards that disease basically madara has adapted and adjusted to the pain, susano is merely the caster's spirit poured outwards which may be why it hurts so much, but it's only the chakra of that spirit think of it as susano as a medium between the spirit and reality, as spiritual chakra.