Interesting question. I found a few benchmarks on zpool mirror that show it roughly 50% faster than a non-mirror zfs configuration. I find that gmirror is consistently slower than a non-mirror configuration .. so while hardware will play a factor (particularly the increased hardware requirements for ZFS), I'd venture to say that ZFS is *probably* faster than gmirror in a well configured system.
If you happen to do some testing, I'd be interested to see your results.
