Ride 9 is a perfect example. Is a rider going to do 3-5 repetitions of the same trail/s in each ride position setting (so 27-45 repetitions), whilst maintaining all other bike settings, and record tangible metrics like power output, effort (heart rate) and time, as well as perceived metrics like comfort, confidence and fun. Record all that in a log book and then crunch the numbers on which setting or settings are best for their riding style/ conditions/ trail type. I think not. Most people don't even have the self discipline to run three repetitions to check a range of tyre pressures let alone the repetitions required to properly set up suspension.
If you do have the self discipline to do all of the above and feel that a geometry change might benefit you (in short most brands do not adjust their rear centre/ chain stay length for frame size increase) then test ride a Norco (they do adjust the effective chain stay for each size) and see if it helps you feel more centred than your current brand. This applies mainly to riders on large or extra large frames as anyone on a medium generally falls within the range of "the average rider for this bike type" considered by the brand at the design and testing stage.
Greg is obviously a candidate for L or XL and is also riding in the 0.05% margin so far above the average rider that every little bit makes a difference.
Feeling naturally centred on a bike is probably the biggest 'game changer' to one's riding.