There's a difference between "noticeable" and "measurable". Honestly, my 70-200/2.8IS delivers the goods every time I use it, as did my former 50/1.4 and 85/1.8 primes. Or at least they do/did when I do my job and get the framing, subject-background distance, etc. right.
Since I don't measurebate or even LOOK at 100% crops, I let the primes go in order to afford a second camera (5D). I rented and fell in love with a 135L over the last week, but I have to say when I was trying to do a kid's portrait shoot I switched back to the 70-200 for the flexibility. Both I and the kid's mom are more than happy with the results, so even though I've wanted the 135 for a long time I don't think I am going to buy one.
Current: 5DM3, 6D, 8mm fish, 24-105/4IS, 35/2IS, 70-200/2.8IS, 85/1.8, 100-400/IS v1, lensbaby composer with edge 80, 580's and AB800's
Formerly: 80D, 7D, 300D, 5D, 5DM2, 20D, 50D, 1DM2, 17-55IS, 24-70/2.8, 28-135IS, 40/2.8, 50/1.8, 50/1.4, 70-200/4IS, 70-300IS, 70-200/2.8, 100 macro, 400/5.6, tammy 17-50 and 28-75, sigma 50 macro & 100-300