Major update:
I forgot that I reset the BIOS, not long before the cooler change. Previously, the system hat a 0.35v undervolt. That had been removed in the BIOS reset.
With the 0.35v undervolt restored, the D15 performance is essentially identical to the iCan dual tower cooler.
I want to get one of the first implants possible. VR headsets are so unsightly.