On the dry , when you see some riders coming down the Alps at 70mph , I dont think they need discs . The only limitation are the tyres . Discs or not if the tyres cant take it , it wont improve braking .
While tyres are a limiting factor the disc pad compond and disc material mean that faster decceleration is obtain than rim brake pads with in most cases more modulation i.e you can get to the limit of tyre grip quicker thus allowing you to get on the brakes later and ultimately be faster in a decent as you spend more time at a higher speed.
In short its the initial bite that disc gives you that slows you faster rather than at the other end where you have almost reached a standstill.
I find I can pull up a lot quicker even in dry conditions using discs and slicks than well set up Vs.
When funds allow, I'm think of going down the Salsa La Cruz route for communting as it's one of the few CX bikes which comes with discs and as there is no CX racing here UCI rules matter little