I haven't had any issues with performance, but then again my requirement was just "reasonable performance".

I ran some quick benchmarks (data below). Obviously this is far from rigorous, but maybe it'll be useful. In previous tests I found that volblocksize=128K was optimal for my stack -- which is why the last benchmarks use that setting.

Every additional ZFS filesystem in the stack may reduce storage efficiency (minimum free space requirements [1]; metadata & checksum overhead [2][3]) -- that's why I used ext4 as the top layer instead of another ZFS.

[1] (as mentioned before) https://www.reddit.com/r/zfs/comments/3vtur4/what_exactly_ha...

[2] https://news.ycombinator.com/item?id=14756360

[3] https://forums.freenas.org/index.php?threads/what-is-the-exa...

  Test setup:
   debian stable
   kernel 4.9.0-3-amd64
   zfs 0.6.5.9-5
   ZFS "pool": mirror with 2x 7200rpm drives

  Benchmark command:
   for i in `seq 1 10`; do sync; dd if=/dev/zero of=DEST bs=1M count=1024 conv=fdatasync; done

  zfs mirror -> dataset
   Data (MB/s): 125,115,104,135,148,170,135,151,118,119
   Mean (MB/s): 132.0
   Std.dev.: 19.9

  zfs mirror -> zvol (volblocksize=8K [default])
   Data (MB/s): 150,115,127,125,122,118,105,118,124,128
   Mean (MB/s): 123.2
   Std.dev.: 11.6

  zfs mirror -> zvol (volblocksize=128K)
   Data (MB/s): 68.5,112,115,114,94.3,85.1,83.1,98.4,120,108
   Mean (MB/s): 99.8
   Std.dev.: 16.9

  zfs mirror -> zvol (volblocksize=128K) -> luks -> ext4  (my stack)
   Data (MB/s): 130,94.4,109,139,138,125,94.9,124,134,133
   Mean (MB/s): 122.1
   Std.dev.: 16.8

edit: formatting