I don't know, if you're running Windows Serve ror Linux, but the Linux drive caches really do a great job when it comes to those little log files, but YMMV of yourse.
As for the rebuild and performance penalties on RAID6: my units, and again I am talking about external RAID systems, rebuilt a 16 drive 7 TB RAID6 in roughly 4 hours, so I really can't see the point there. The raw read/write performance is 175 MB/sec vs. 160 MB/sec over FC, so that seems to be reasonable as well.
In the end I think that you have already made your mind up, so why don't just go for it? Obviously nobody screamed out at you for sugegsting something silly or dump.
When it comes to tuning and setup nobody will ever garantee that a particular suggested drive/volume layout will work or meet your demand.