Not to mention that 300MB/s is considered best-case conditions for sequentially stored data. Anything even remotely random is going to have far slower archival

Not to mention that 300MB/s is considered best-case conditions for sequentially stored data. Anything even remotely random is going to have far slower archival times, unless you have archival software that does parallel I/O. (In my experience, not many archivers do this.) Most production tape installations include dedicated disk storage to act as a buffer for the drives. You archive to disk first, so that your drives aren't tied up, and then write the now-sequential data to tape.