Nice testing...good to show sandwiching isn't always a free lunch.

Would like to see more higher RPM testing though for a couple of reasons:

1) air saturation levels of most radiators decline as airflow increases (meaning the air leaving the first radiator still has a decent potential to absorb more air)

2) static pressure of a fan scales with the square of the RPM. Airflow scales linearly with RPM. At higher RPMs, fans should have little problem moving air through the sandwich The increase in static pressure from increasing RPM is way greater than switching from a 25mm fan to a 38mm fan. Though the right low-speed 38mm fan could be a decent middle-ground (aren't many out there though )

Of course though, showing a lack of efficiency at lower RPMs is critical to knowing how well it performs too though

p.s., I like the idea of the airbrake testing!