Prepare yourself for strange logic.
I found that a single SMP client on my machine never reached 100% CPU utiliisation. Sometimes it could be as low as 80%. Which means that anywhere between 5 and 20% of my CPU wasn't being used.
So, I wacked another SMP client on there and now enjoy a slightly higher PPD than I would with a single client.
The reason being is that with two WUs running they both use 50% of my CPUs power. Not just 40% if you could somehow half the requirements of the SMP client. So as you can see I can get up to 20% more output if I have two WUs running.
Understand?
As said though it does eat lots of RAM. The clients are currently using over 450MB of RAM between them.