Please consider zstd instead. It is superior in every way except memory usage and compatibility with old systems.
(Actually this problem is why I haven't owned a tape drive for a while.)
I'll get a small benchmark script for that done over lunch. Come back here later.
Is this a continuation from previous message ? Because multithreading mode of zstd doesn't impact its compression ratio. It's actually an interesting property of zstd : whatever the nb of threads used, the compression ratio remains the same (aka. reproducible).