Its true, Until you do the conversion, there is no way to know what kind of gain you will get. I have seen No gain in a few Improved 06's to as much as 200 fps. Same for the 25-06 which is already overbore.

I would never be willing to bet money on the outcome of these conversions. Like you said, the 250 Savage and the 22-250 seem to be exceptions and case life in the 22-250 is probably well worth the conversion as it usually stops case length growth. IME