verdi.11, you are assuming that the spell damage is multiplicative, but it is actually additive (this is what besty was talking about). Basically the +100% spell damage you are gaining is added with ALL OTHER sources of + spell damage including attributes, passive tree nodes, gems, armor and weapon affixes. When you already have +1000% spell damage from those and are doing 20-35 k damage, another +100% spell damage is really only a 10% increase (1000+100=1100, 1100/1000 = 1.1 or 110% damage from where you started at). This means that any of those additive damage passive nodes are a lot less valuable (usually only 1-1.5% damage increase), and generally the same goes for affixes on armor (though the percentages that roll are much higher).
It sounds like besty was assuming that when you traded catalysts, the old catalyst had a +# to spell damage on it. As he mentioned, these affixes are WAY better for increasing your damage, as they increase the BASE damage, which all of the affixes and nodes mentioned above are just multiplying the base damage.
Since you were looking for parts of the game that do multiplicative damage (takes the total damage and multiplies it instead of only base damage), this is a great video posted by Binashole aobut which passive nodes are multiplicative: https://www.youtube.com/watch?v=MfvZnSuqTqM
From my understanding of the game (or any ARPG), very little is multiplicative damage, as balancing those types of calculations becomes a nightmare. I can't think of anything else in the game that does multiplicative damage, except maybe the skill Mark of Impurity.
