In case you're curious I tested both the 80MB/s and the 45MB/s on a D600 today and noticed ZERO perceived increase in speed for both for writing to as well as reading from the card.
Using Continuous High speed mode (which to be fair is something I never use in real life) I was able to get 11 shots in quick succession after which the shooting rate slowed to an approximate and consistent single shot every 3/4th of a second while the buffer cleared. Read speed back from the card such as when sharing back of the camera previews with clients was also indistinguishable between cards.
I then switch to Continuous Low speed mode (set to 3 shots/sec) which I actually will sometimes use when working with kids and occasionally at weddings and I was able to get 15 shots in before the shutter slowed to the same single shot every 3/4 of a second.
Unfortunately I have to conclude that the faster rated card is not worth the premium of $20.