I can only say that I saw significant performance difference when CPU threads were saturated while running BRP tasks. Thus freeing additional CPU core(s) has helped much.
You can make a simple test: watch the CPU usage of BRP tasks while running different amount of other CPU tasks - suspend them all, then resume one by one.
Once you get to a point where the CPU usage of the GPU task starts to drop, you know the CPU is saturated and that is going to hurt your GPU WU performance.
I'm using BoincTasks which shows this, you might also use other tools.