Hi,
Sorry for my late reply. I guess you are referring to change the function used in the TMVAGui to draw the efficiency and significance.
Instead of using the GUI you can call directly the corresponding function, where you can change the function used too optimise.
For example for the output of TMVAClassification, you can call this function:
TMVA::mvaeffs("dataset","TMVA.root",1000,1000,true,"RooStats::AsimovSignificance(S,B+0.01)")
If I want to use the Asimov mean significance instead of S/sqrt(S+B)
Cheers
Lorenzo