Out of curiosity, last night I turned off flex and tried Rise Time=1. I didn't notice any difference in comfort. A couple of brief CAs snuck through, but that was probably a coincidence. And as I just noted, my average EPAP pressure was still below EPAPmin. I'll leave it on Rise Time=1 for a few days and see if I notice anything.
I'm not trying to say that this is a problem in need of a solution. I'm just trying to understand what's going on.