For a long time I ran with all clients at 255. This ran well for a period, but after introducing other things into the mix, by which I mean other directx instances, applications etc. I began to have issues with my clients choking.

I changed out to 255 for the slaves and 15 (first four cores) for the main and ran that config for a few weeks and again, found my experience smooth on all clients.

The choking returned when I started playing alot of h264 media in the background, which is considerably cpu intensive. I decided to try the setup I recommended in the OP, in order to offer the operating system some resources to play with and have had success with it thus far.

I would conclude at this point that my findings have been that spreading the load across all cores is a good move for your wow clients, but this has an adverse effect when adding a cpu-intensive application into the mix. The key, I believe, is hitting the affinity balance point between distributing wow client load and leaving enough readily available resource for the rest of the system.