Casey Butt's research has not specifically been replicated, but there have been numerous studies investigating the Fat-Free Mass Index of athletes from different sports—and they would appear to validate his findings.
For example, a study of collegiate athletes found that they had an average height-adjusted Fat-Free Mass Index of 22.8 kilogram metres (kg.m), which if we extrapolate from comparisons of collegiate and professional athlete average sizes in basketball and American football, would be consistent with the ranges we might expect from Butt's research.
“Also, all of the men were bodybuilders, who one would assume aren't the best athletes around, they care about building looks not muscle.”
Muscle is the singular preoccupation of bodybuilders, and the very foundation of the aesthetics that they are striving for. Bodybuilders, more than any other athletes, care about muscle, and bodybuilding training protocol is optimised for muscle building. Athletes of other sports, by contrast, are concerned primarily with performance—not muscle bulk.
Thus, it is very reasonable for us to assume that bodybuilders would exemplify our human limitations of muscle growth. And that notion is supported by comparisons of Body Mass Indices (BMIs) of professional bodybuilders with basketballers, baseballers, hockey players, and American footballers. Indeed, the BMI of the average professional bodybuilder is comparable to that of professional footballers—the heaviest of the four—with the heavier bodybuilders being well beyond the outliers in football, relative to their heights. Interestingly though, professional sumo wrestlers have been found to have significantly greater muscle mass than bodybuilders, since they are not subject to such limitations of aesthetics or specific power requirements.
So there is certainly scope for further research, especially with regard to the limitations of drug-free training. And the formula might likely be improved by the inclusion of data from professional athletes from power sports, too, which may indeed alter its parameters slightly. However, those athletes should already be represented to a degree by the data examined by Butt, and precisely the reason that the formula contains fields for joint circumference.
The existing evidence seems to corroborate the general findings of Butt's work—that our potential to develop lean muscle is dependent upon our height and genetics, and that those genetics are expressed by our bone cross-sections, and hence metrics such as wrist and ankle circumferences.
I hope that helps.