While ZFS's RAID-Z2 can offer actually worse random read performance than HW RAID-5 it should offer much better write performance than HW RAID-5 especially when you are doing random writes or you are writing to lot of different files concurrently. After doing some tests I happily found it exactly works that way as expected. Now the hard question was: would RAID-Z2 be good enough in terms of performance in actual production environment? There's no simple answer as in a production we do actually see a mix of reads and writes. With HW RAID-5 when your write throughput is large enough its write cache can't help much and your write performance falls down dramatically with random writes. Also one write IO to an array is converted to several IOs - so you get less available IOs left for reads. ZFS RAID-Z and RAID-Z2 don't behave that way and give you excellent write performance whether it's random or not. It should also generate less write IOs per disk than HW RAID-5. So the true question is: will it offset enough to get better overall performance on a production?
After some testing I wasn't really closer to answer that question - so I decided on a pool configuration and other details and decided to put it in a production. The business comparison is that I need at least 2 HW RAID-5 arrays to carry our production traffic. One array just can't do it and main problem are writes. Well, only one x4500 with RAID-Z2 seems to do its job in the same environment without any problems - at least so far. It'll be interesting to see how it will behave with more and more data on it (only few TB's right now) as it will also mean more reads. But from what I've seen so far I'm optimistic.