A user mailed the list and made the point that perhaps superposition in RAW should be error weighted. It wouldn't be too hard to do. Use something like scipy.optimize.curve_fit to fit the superposition with errors. Maybe have a setting that toggles which one you use.
They also wanted to calculate the chi^2 between curves. There's no way to do that now, but maybe it's time to finally add chi^2 as a comparison method in addition to cormap. Then you could get this value from the similarity window.