Tim was nice enough to reply to my question on his blog.
Here is the poop for anyone else facing this issue:
Tim says:
Sure! A ways back I did some testing and did not see a performance benefit to the read cache when in a raid 5. When information is read, it is being read from multiple spindles. So, in a 6 disk raid 5, we have 6 disks to read from each at its max transfer speed. The odds of any of those blocks being in that 25% of cache we dedicated is rare, so moving it from disk to cache to use doesn’t save us any time. However, when writing – it is much quicker to write to the cache, and then allow the controller to flush to the disks and calculate parity. Since you receive basically no benefit from the read, I choose to dedicate all to write cache, where the benefits are obvious.