I do not understand. Why are you excluding all DP machines for OS 9 completely? OK the system isn´t using the 2nd CPU but some applications like Photoshop can.

Because DieHard said

Users that are OS9 only may choose a single CPU; depending on the DAW host application, a single will run both cooler and more stable (with some OS 9 based applications), so the optimal CPU (without overclocking) is the fastest single with the most cache...

and I was assuming that the OSX+OS9 reccomendation by DieHard can be used for OS9 Dual CPU usage, too - i.e. the recommendation for OS-X + OS 9 would be the same as for a User who e.g. uses Photoshop in OS 9, only.
(is it "was assuming" or "assumed"?)