I should have been more clear.
I have seen multiple times in the past where small seemingly innocuous changes for minor bugs have introduced critical bugs. In this case TabletServer uses the Stat class, but does not use the std deviation. The risk is a small possibility of introducing a new critical bug in tserver if the change breaks Stat in some strange new way. The benefit of the change is that informational output from a few test may be better.
Based on this comment, I do not understand which version you would support including the new implementation of Stat in. I also do not understand if you would support fixing stdev in older versions while leaving everything else the same. FWIW, my implementation and the commons-math implementation were the same (theirs was a bit more general and better documented, but otherwise identical).
The reason I asked for clarification is that I cannot tell if you are -1 or -0 on including the full change in 1.5.x.
Something else that would cause problems would be if the Apache library stored all of the data you were computing stats for. I assume it does not do this, but would have to inspect the code to be sure.
It does not store the data, so this is not an issue.