Back on the topic of allowing fans to warm up. Let it cool for 45 mins.

On startup: 1305rpm
After 1 min: 1315rpm
After 2 mins: 1325rpm
After 3 mins: 1335rpm
After 4 mins: 1350rpm
After 5 mins: 1355rpm
After 10 mins: 1370rpm
After 15 mins: 1400rpm
After 20 mins: 1415rpm

Voltage was monitored the entire time, and remained consistent. The fans just warm up and vary in speed. 110rpm is a very significant difference. That's 4-5cfm through the radiator, and it's no wonder I was pulling my hair out wondering what was going on.

This exactly explains why I wasn't seeing repeatable results before. Things were changing even over a period of 20 minutes.