I’ve long followed Tom Anhalt’s writings on this topic. He uses rollers for testing, which are vastly smaller than the drum used by BRR and he comes up with similar rankings. I’ve done similar testing myself with rollers and a power meter and my rankings have come up similar to his and BRR, and match reasonably well with my subjective perception of how the tires roll on very smooth asphalt.
It’s an interesting assertion that manufacturers stack the deck with regards to performance on the RR tests. It’s true that a tire that’s wide relative to its labeled width will roll faster at the same pressure on a drum test, but a wider tire is also heavier and I’d say that metric is given at least as much attention, if not more, by the general riding public as rolling resistance tests.
IIRC the GP4K was actually only a moderately fast rolling tire in these tests and significantly slower than the 5K. It wasn’t the most supple tire but it wasn’t some sort of garden hose-stiff tire either. If stiffness were the only factor in determining how a tire fared in drum tests then tires like the Gatorskin would do very well in drum tests and we see that’s not the case. I suspect that part of that much-touted suppleness attribute is not just casing flexibility but also a softer tread compound, which may add grip and a nicer ride at the cost of rolling resistance.
Tom Anhalt has asserted that the Wheel Energy rig - despite adding damping - still only has approximately 10% of the damping of a human body, and this is borne out by their tests of gravel tires where the breakpoint at which rolling resistance increases is much higher than any sensible real-world pressure for those tires. So I’m not sure the damping really adds that much useful information.
I agree that drum tests do provide some useful data as long as you understand the limitations and any quirks of the testing protocol. They are reasonably predictive of hysteresis losses and rolling resistance on smooth pavement, but not predictive of the effects of tire width, pressure, and tread on rolling resistance over varying and rougher surfaces.