I actually made an addon called
NerfEverything that does this a while back, it's written pretty terribly and will almost definitely taint your interface, but you can look at how I did it if you want.
Since then I've thought of several ways this could be done without tainting the interface and with much less code but some text simply can't be modified by an addon, namely the scrolling combat text above units heads. There isn't really any way around this (you can hook onto nameplates and sort of fake it but it's pretty limited).
Also I wanted to use a logarithmic scale instead of a flat percentage reduction but for a reason that escapes me right now (it was a pretty good reason at the time) I couldn't get it to work how I wanted it to.