Remounted the block due to a faulty initial mounting and re-ran tests with a very low flow rate.
The flow rate was cut down to 1.0lpm (0.25gpm). I can't imagine people running with flow rates any lower than this.
I used a Morgan-core Duron (for its extremely high wattage density being notoriously hard to cool) running at 1200MHz/2.15v putting out 81.4W according to the
ComputerNerd thermal calculator running BurnK7 to generate full thermal load.
The results (applicable only to my setup):
Eheim 1250 @ 6.8lpm => 20.9/23.1/41.0 (air/water/cpu die)
Eheim 1250 @ 1.0lpm => 20.9/22.9/43.0 (air/water/cpu die)
This is somewhat better than what Les's graphs were predicting.