I appreciate all the responses on this thread, even though some are conflicting :). We are going to try these one at a time, but we'll likely need a day or so inbetween each to see what impact (if any), so it will take time. But I will post back here our findings.
We'll start with dirty_background_bytes, as that's straightforward. (dirty_bytes is already set to the same size as our RAID cache.)