If I have two machines both 3rd Generation AMD Opteron, one Shanghai & one Istanbul and I can vMotion between them. Does it buy me anything to enable EVC at the 3rd Generation AMD level? Will I lose any CPU features?
My thoughts are, I can enable EVC 3rd Generation with running VMs and I can't enable 2nd Generation EVC with running VMs. I figure there won't be a lose in any features because if there was a reduction in feature set then I couldn't enable EVC with running VMs.
You will gain features by enabling EVC for your 3rd Gen processors. With EVC, you want to enable the lowest level that your processors support. If you had 2nd Gen and 3rd, then able 2nd. In your case enable 3rd if you are planning on not introducing 2nd Gen later on.
If 3DNow! isn't needed (only for 3D applications) then set EVC to Gen 3 (no 3DNow!). This will give you both Gen 3 and Gen 4 support in the future.