He also played much more than the others.
I think I've read that in the last part of their tour with Kramer he won 15 out of their 32 matches, which means he won more in this last part than in the first parts and almost half of these matches. This shows he really got to another level. So perhaps he could have been the better of Kramer in the following years. We'll never know.
I agree! Probably the hardest thing for the newcomers was playing on the indoor surfaces, which were very fast. One had to be a very good serve and volleyer and get used to the footing. Rosewall played Gonzales pretty evenly once they got onto grass and clay. I have no doubt that Poncho became as good or better than Kramer the more he played the Pro Tour.