RAID6 can perform optimized writes.  Each member in the raid stripe is still 4 tracks (256k).  So for 6+2 that is 24 tracks, and 14+2 is 56 tracks.  One thing to be aware of is the effect that virtual provisioning may have on this.  We allocate 12 tracks per raid group when you write to a thin device.  This isn't enough for a full optimized write.  So if the DA starts to destage before the rest is written, it can't be a full optimized writes.  In some cases the DA may read the missing data to fill the stripe, both on RAID5 and RAID6, because that could result in far fewer disk operations than if we have to do the full XOR from disk.
Trying to understand more about raid-6 write relative to I/O size.  Does the DMX do any raid-6 performance optimizations similar to a raid-5 full raid stripe write?  From my reading of the oracle whitepaper you can do a straight raid-5 write to disk with no read of data if the write transaction is large enough.  3+1=768k (3*256) for 7+1=1792k (7*256) size of a sequential write (for a DMX3).  Reading on raid-6 I can't find any similar statements on direct write... but if I apply a similar logic to raid6.  From the docs I have found it appears that raid-6 uses a size of 64k per disk rather than raid-5 256k (but that seems very odd so I'm not sure on that).  Assuming that's correct would a aligned sequential write to a 14+2 raid devices size of 896k (14*64k) be a straight write to disk with all of the raid calculation happening in cache avoiding having to do a read in from disk... or more likely am I in left field waving my hands in the air in a completely wrong direction?
DMX4 Raid-6 & full raid write