Hmm, interesting take on the added weight of WE shitmetal slides needing CO2 to propel them - perhaps that's why I never found shooting my WE to be very satisfying when I had it.
CO2 will likely tear apart your internals, sooner or later. Your gun doc likely suggested a new loading muzzle (good idea as WE ones kinda tend to break anyway), and perhaps a new piston head. New piston head isn't entirely necessary, but I wouldn't trust a WE potmetal part to take the constant beating of the full brunt of CO2 pressing against it to blow-back the slide. You may have been told about a new floating valve/bulb/whatever-else-its-called, and a new valve blocker - I think these would be a good idea, as well. CO2 mags might need a new hammer spring in order to properly open the valve every time (I stress the might since I can't remember what WE hammer springs are like).
I agree that propane is easier.... lol.