If the flow through the radiator is higher, the pressure drop must be higher too. It is possible that the pressure is altering the radiator physically, albeit very subtly? Maybe making the tubes flex outwards a little, which would make the tubes' inner surface/CSA ratio worse?

You couldn't have waited another week or two to post this, Martin? I'm halfway through building a cabinet to house three triple rads, three pumps, and I was planning on running sub-600rpm fans. This is throwing a monkey wrench into my plans, darn it.