> the AMD system has the additional disadvantage over the others of being 1P instead of 2P, which approximately halves the available cooling and correspondingly limits clock speeds.

This doesn't make sense to me. I'd assume each system to be spec'ed correctly for the TDP of the processors. Each system has it's own HSF and case solution.

Multiprocessor systems do perform as less than 2x of their single processor counterparts, but it nothing to do with thermals in a properly designed system.