On linux: following 110 seconds of extracting pkgsrc.tar, an immediately subsequent umount command takes ~10 seconds to complete. So, while interesting, this adjustment clearly can't explain the order of magnitude difference (~6.7 MB/s on Linux vs. ~0.7 MB/s on FreeBSD and NetBSD).
Thanks for the interesting suggestion, nevertheless; the thought does make me a bit wary of accidentally power cycling before sync finishes. Although, I suppose that is why we have journaling.
Another interesting thing to note is how aggressively linux consumes ram when copying files.