Just a few groups like these could easily support an order of magnitude increase in the user base without creating a negative user experience on Steem

Maybe. Currently it costs a certain amount of RCs to create a post, a certain amount to vote, etc. These numbers are not fixed. If there is an order of magntude more demand for resources, these prices will increase, regardless of where that RC is coming from. These parameters were set, roughly, based on sustainable resource usage rates for blockchain sustainability. While some progress has been made on scalability and sustainability with MIRA, its numerous limitations mean that doesn't seem to quite get us there, yet.

How much headroom for increased usage is there within the current physical blockchain scalability without significantly increasing the RCs required for each basic operation, which is essentially two steps forward, one (or two?) steps back?