"It depends"
Without caching in any form zfs will be slower (using zil on the hdds) than normal raid 6.
With the use of ram as cache + an extra slog device raidz2 will outperform raid 6.
With ssd caching (cachecade or maxcache) raid 6 can get similar results in sequiental reads/writes, but the ram caching + slog will make zfs faster for small random io