Testing Received Wisdom: Never compress twice?

Results of compressing the factory image* of a new 3TB drive with various methods:

MethodSizeTime
gzip --fast16.7 GiBKept up with the drive
gzip --fast | gzip --fast501 MiBKept up with the drive
gzip --fast | gzip --fast | gzip --fast426 MiBKept up with the drive
gzip --fast | gzip --fast | gzip425 MiBKept up with the drive
gzip --fast | gzip444 MiBKept up with the drive
gzip --fast | gzip | gzip425 MiBKept up with the drive
gzip --fast | xz426 MiBKept up with the drive
xz --fast833 MiB3 days
xz --fast | gzip 418 MiB3 days
xz --fast | xz --fast 416 MiB3 days
xz --fast | xz 408 MiB3 days
bzip2 --fast3.24 GiB38 days
bzip2 --fast | bzip2 --fast556 MiB38 days
bzip2 --fast | bzip2445 MiB38 days
snappy133 GiBKept up with the drive
snappy | snappy6.80 GiBKept up with the drive
snappy | snappy | snappy856 MiBKept up with the drive
snappy | snappy | snappy | snappy583 MiBKept up with the drive
snappy | snappy | snappy | snappy | snappy580 MiBKept up with the drive

* Why compress a factory image?

To put it back when I'm done with the drive. If for some reason I sell the drive or return it for warranty service, I'm certainly not leaving my stuff on it, and it seems a teeny tiny bit rude to pass along a wiped (all \0s) drive, especially to a less tech-savvy person. Presumably someone finds value in the junk the manufacturer takes the trouble to pre-load the drive with.

So I stash it in case I don't run the drive into the ground myself. The cost of this is tiny: 0.015% in this case. Compare: The ext4 overhead is 0.12%. The default mke2fs -m reserved-blocks-percentage is 5%.