The data rate in the edit control you see when you click Ctrl+slider was what I
was referring to. That rate is not the same as the array's raw sustained
thruput rating. I called it a "QuickTime data rate" for lack of a better term.
As you've discovered, you just have to spend some time tweaking it to find the
optimal rate. It really depends on what your content is, and what output
quality you want.
– A.J.