if you went from factory 15/46 to 16/45, then it should be 91.7% of rpm,** Speeds are not what the gearingcommander.com site predicted.
That fits your 6000 rpms speeds.
But 5000 rpms should become 4585 rpms.
If your rpms go down to 4250 (85%), you'd have to have about 16/42.
So gopro is quite off!