Hi Andy. IBM is the vendor. They have a very thorough whitepaper and published presentation on why they want us to use CPU affinity. They saw dramatic performance gains (or rather prevented performance drops) when using it. Right now they have a physical server with 4 quad-core processors with hyperthreading so they see 32 CPUs. They are sure that their jobs are CPU bound and it directly correlates to the number of cores. We have a ESX host with 2 eight-core processors in it. I convinced them to allow me to show only 8 cores to the OS for now but it is very likely they'll ask for more. If they run their jobs and they go faster with more cores they'll ask for more. The consultant they brought in confirmed this requirement for CPU affinity as well. Unfortunately as a jack of trades I'm not in a position to intelligently prove they're wrong or right without significant testing.
That's good to know about DRS migrations. We don't plan on using it but it's good to know.