but if you can transplant the plant before it's completely rootbound and still get a much denser root system is that not peaking the plants performance?
nope, to get peak performance, watch a plant with unlimited root space, perfect supporting cast, even and adequate moisture, aeration, etc...
limit root space and you will limit performance.
if you are transplanting so early that the pot is not starting to fill, isn't that just a stressful way to simulate not transplanting? the real deal will be better.