Education: Why is Performance per Millimeter a worthwhile metric?

Typically how we control leakage is,

(a) we go for the target frequencies we want to hit
(b) replace the cells in paths with slack with low leakage cells

Obviously the process is iterative (in terms of achieving balance between leakage and perf and that discussion is for another day). In step (b) above, cells with with equivalent drive strength but lower leakage tend to be larger than their leakier counterparts (but you all knew that already ) ).

From my experience, we can expect anywhere from 5% to 20% increase in area (largely a function of timing slack you have and leakage requirements).

I'd say perf per mm^2 is more a function of the choice architecture than floor planning and cell designs (based on my experience in a very niche area). Designs that tend to move large chunks of data across the length and breadth of the die tend to be less efficient in perf/mm^2 (again based on my experience).