Sun apparently had internally done most(?) of the work on BP rewrite (as it's referred to), but performance sucked rather badly, and the work has not been shipped by Sun/Oracle, let alone anything that came out before the F/OSS code sharing ceased. 
(Performance sucked, in particular, for similar reasons to why dedup performance has such a high penalty on ZFS - they end up storing a lookup table that gets hit for every block that gets written after you turn on dedup, and so if that table stops fitting in RAM, it's a bad time - let alone the issue of making sure the disk backing the DDT storage is sufficiently performant...)
 - http://www.listbox.com/member/archive/182180/2012/01/sort/th...
Nowadays, RAID5 is deficient.