Thats not a diminishing return. Youre gettting to that mistake by expressing the result by looking backwards...
+ 1 disc = +100% damage
+10 disc = +1000% damage
+50 disc = +5000% damage
It doesnt diminish no matter how high you stack disc.
CDR however is a diminishing stat and to save me some effort its explained better than i could ever try right here
However i did make a mistake above by saying it has to be additive and a %age - looking at the reddit explanation its just a multiplicative calculation.