Yeah, sorry, I meant to say ignore the driver advice for the 780s, but I forgot. 332.21 is the best driver for GK110 (780, 780 Ti, Titan). I think this only applies to core 17 WUs, with core 15 not really caring which drivers you have.
I think CPU:10 should be enough to keep the GPUs sufficiently fed, but I'm not positive. When I added my second 780, I stopped folding on my CPU altogether. This is only a problem when running core 17, which due to Nvidia's not-so-great OpenCL implementation needs a thread/core per GPU. Core 15 uses virtually no CPU time.
You are correct, the core 15 WUs you're currently running need to finish before you'll get core 17 WUs. A given project will only run on one core. At this point the only core 17 project in wide circulation is 8900.
The reason you're not seeing those numbers is because core 15 PPD is terrible in comparison to core 17's. Core 17 uses the QRB that CPU projects have used for years now, while core 15 does not. Core 17 is also more scientifically valuable than core 15, so the points should be higher. At best each of my 780s gets around 60k PPD on certain core 15 projects, while others can go as low as 31k.