I do not have Techstream (yet), but I'll certainly look into it. It sounds like Techstream would allow me to determine if the ECU is controlling the fan properly. So, that would tell me if the problem is before or after the ECU.
I was also looking at the BRZ wiring diagram (
https://www.ft86club.com/files/BRZwiringi.pdf - page 338) last night and from it I'm gathering that the fans are on / off and not speed controlled (which is what I assumed) as the motors are just connected to relays. So, perhaps the difference I'm observing between the BRZ and Impreza is that the BRZ has higher performance fans than the Impreza because racecar.
So, sounds like the next steps tonight are to look into Techstream. I'm also going to scour the wiring diagram to find the best place to unplug the AC to see if that shuts off the radiator fans. Might be a reverse (poor man's) test of what Techstream would tell me.