I would caution that long term stunted populations select for small size in many species. It is a survival adaptation selection process.
Shetland ponies and Mammoth and Mastodons isolated on islands come to mind. On the other hand, I am aware of no research documenting this effect with LMB over time scales of a human lifetime. Please point me in the right direction if you are aware of one.
My sense of LMB is that there is considerable adaptability already built into their genetics for environmental variation of food availability and competition. LMB from an environment where food is abundant and competition is low (they go together) have the genetics to grow slow as an adaptation to environment where they may have been stocked at too high a density. The opposite is also true. There is intense predation pressures for fish populations. I don't see this factor favoring slow growers or smaller fish so I think more than one factor would influence selection and that meaningful selection for slow growth or smaller ultimate weight potential probably would not rapidly occur (In other words, I think it would take more generations than Fyfer's fish have encountered).
I doubt there exists any water that grows LMB at their maximum genetic potential over time. Competitive pressure and food supply always limit growth below this potential. For genetics to contribute meaningfully to growth and ultimate weight, the competition for resources must be subdued enough to allow genetics work their magic.
