Yeah, it's an easy misconception. You'd think that something has to be best right after the pump and it is, but it's pressure only. The reason pressure doesn't have any effect on how well it cools is because water and most liquids by nature are incompressible. If water was compressible, the higher pressure would result in a more dense fluid which could then have better thermal transfer ability. Theoretically there is some truth to that, but the amount is so small you can ignore it.
If water was compressible I would take a T line and extend it up my wall as high as it could go, this will effectively put the entire loop under more pressure. Unfortunately it's not, so the only thing that would do is look a bit funky and perhaps stretch your tubing out ever so slightly...in the end not enough to make a difference.
What does make a difference is the temperature variation throughout the loop..it's still only a few degrees C at best, but it's at least measurable.
Thanks for sharing your real world results though, that's what matters the most