Mike, as I am sure you are aware, Haddo (or Keithfield) was a Brown trout only water when Shell originally had the lease. I think they started to stock after a few years though.
I believe most of the Brownies were stunted things, which I guess proves the water is more than suitable for Brownies.
How about sticking in 10 big carp as well...they are having fun with Carp on the fly at Glen of Rothes
As for fishing the place, I love Brownies, but from a financial view, could you survive? How much would a ticket be?