A few thoughts from my experience with anthropometric requirements on F-35, which might be of interest:
The JSF programme was the first (as far as I know) to have the '97%' requirement, and meeting it was non-trivial in many areas. Martin Baker did a fantastic job designing an ejection seat that could cope with a very large range of sizes and weights, but the '97%' requirement led to a much larger number of required rocket sled test runs to certify the various seat/helmet combinations across the size/weight range.
The requirement also led to a number of redesigns of the integral boarding ladder to provide a footstep low enough to the deck to allow the shortest pilot to be able to reach it. It also proved quite hard to find someone short enough to carry out the required demonstration that the new design worked. Having designed a footstep low enough to meet this requirement, we then had to find a way of locking the lower portion of the ladder up out of the way to ensure that it didn't foul a towing arm when the aircraft was being moved around.
There were also a number of issues with location of servicing points, panel fasteners and so forth, with redesigns needed to locate them low enough to be reached without using steps or ladders. In my view, (and that's all it is) imposing the requirement was definitely the right thing to do, and it made the engineers work harder to challenge old assumptions.
Best Regards as ever to those working hard to meet the requirements, whatever the requirements are,
Engines