I just ran my RAW test from yesterday again after updating to the newest LR classic even on the 13" (and rebooting it this time before the test)
13": 11 min 4 sec (avg. clock speed 3.5)
16" i7: 2 min 47 sec (avg. clock speed 3.5)
16" i9: 2 min 26 sec (avg. clock speed 3.09)
Concerning the fans: This time they came on at about the same time. Maybe the i9's kicked in a little earlier, but not much. Noise wise, they're the same. I lifted each of them to my ears in turn, a couple of times, and I couldn't tell a difference. When they are running at max power, they are significantly louder than the 13". But I find the noise acceptable.
I also managed to plug in my external monitor. Neither machine spun up the fans in clamshell mode. But as I said, I only have a FHD display, that might not put enough stress on the laptop.