More capacity means more cache - the more data can read server from cache instead of getting data from disks, this is improving performance. It's fast, effective and safe.
To know the metric - you have to know what storage blocks are often used when random read-write occurs (mostly, but it may depend on data bandwidth too). If you are accessing terabytes with constant gbps bandwidth - it may not have effects, even in case of random writes.
It's not only about the read, depending on your configuration it might be write cache too, but if write cache mode is not battery-protected - you'll lose non-written, active data.
HDD's also have a cache memory, which can be disabled too (ex. write-through mode).