The pump energy is only part of the answer. The difference in power of those two pumps is not enough to raise the water temperature by 4°C. Not unless your true flow rate is on the order of 10 gallons per hour anyway.
What other temperatures have you measured? How are you defining "under load" for the CPU?
Normally the culprit is crud plugging up the radiator fins, changes in room air temperature (as much as 2-3°C from floor to desk level), air in the systems, etc.
After all that blah, blah, blah, I wouldn't personally expect more than a 1°C change if all that changed was the pump (and correspondingly the flow rate).
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