I was a dealer for Nelsen for many years and at one time before Watts bought them, for Alamo Water. I can't say I ever saw anything about a minimum gpm size for residential softeners. And for many years I sold many 1.5 - 4 cuft softeners with no customer complaints. That is because they were sized and programmed to regenerate every 8-10 days.
How about you show us the data instead of just making the claim?
In another thread about sizing you say there usually is no problem with the larger size and that it is the right size for the peak demand requirement of the house and that if the peak demand is exceeded, the customer gets hard water through the softener. Now here you don't mention that and that makes your advice questionable at best.