If you are talking physically, 2TB/8TB would theoretically be faster than a 2TB/2TB disk if the performance attributes were the same. But 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.
A quick google search shows that there are marginal gains on the outer track vs the inner, but that is only on sequential workloads. For something like GitLab, the workloads would be anything but.
http://superuser.com/questions/643013/are-partitions-to-the-...
1. If I look at 2TB vs. 8TB HGST drives, their seek times are 8ms and 9ms respectively. But if you're only using a quarter of the 8TB drive, the drive arm needs to move less than a quarter as much. Won't that save at least 1ms?
2. The 8TB drive has a lot more data per inch, and it's spinning at the same speed. Once a read or write begins, it's going to finish a lot faster.
3. Here's a benchmark putting 8TB ahead in every single way http://hdd.userbenchmark.com/Compare/HGST-Ultrastar-He8-Heli...