> a 2TB HDD will have a faster average seek time than an 8TB HDD due to physical design. Any performance gains of only partially filling a drive would probably be offset by the slower overall drive.

I'm skeptical here. So your minimum seek time goes up on the 8TB because it's harder to align. But your maximum seek time should drop tremendously because the drive arm only has to move along the outer fifth of the drive. And your throughput should be great because of the doubled linear density combined with using only the outer edge.