Goosen also reveals that the Xbox One silicon actually contains additional compute units - as we previously speculated. The presence of that redundant hardware (two CUs are disabled on retail consoles) allowed Microsoft to judge the importance of compute power versus clock-speed:

Every one of the Xbox One dev kits actually has 14 CUs on the silicon. Two of those CUs are reserved for redundancy in manufacturing, but we could go and do the experiment - if we were actually at 14 CUs what kind of performance benefit would we get versus 12? And if we raised the GPU clock what sort of performance advantage would we get? And we actually saw on the launch titles - we looked at a lot of titles in a lot of depth - we found that going to 14 CUs wasn't as effective as the 6.6 per cent clock upgrade that we did.
Digital foundry vs the Xbox One architects