Just month ago a friend bought a road bike with disk brakes so we decided to test the stopping ability of his vs mine with the Koolstops, he felt for sure his were going to be better. So we did 3 test at 20 mph and 3 at 25 mph, in all
6 tests we were equal to within a foot, sometimes I stopped quicker and sometimes he did (which I'm sure is due to reaction timing) but never was either of us more than a foot difference. While it wasn't raining when we did the test, perhaps in the rain he might have a slight advantage, but not sure if that's true since the KoolStops have an instant response even when wet compared to Shimano which were slow to react when wet; I am also pretty sure from my experience with Shimano pads that had the test been done with those pads on he probably would have stopped quicker in all the tests. So yes good rim brake pads like the KoolStop Salmon makes a big difference dry or wet.
What kind of disc brakes was your friend using ? Cable actuated or full hydraulics ? There is a big gap in modulation and control and to some degree power between the two types.
He had hydraulic, can't recall the model, some number he rattled off, but they were Shimano, but he had Shimano Di2 Ultegra so I can only assume the brakes were of that level as well since he said he decided he wanted full Ultegra when he bought his new bike.
Keep in mind the test that we did was on my bike equipped with 105 brakes, and my wheels were heavier than his, he had Ultegra AL wheels and I have the lowend RS501-30 AL wheels, so even with my heavier wheels the stopping distance as indicated. His bike overall was about a pound less than mine and he weighed darn close to me from what he said, so I don't think weight was an issue, not sure if my wheels weighing about 400 grams more than his would make a difference as long as the total weight is about the same. He was using Conti GP4000s 700x25c all around, I was using Hutchinson Intensive 2 tires 700c x25 on the rear and 23 on the front. Again without a full on scientific study of the exact weight, various tire differences including PSI, and wheel momentum, one riders experience at stopping vs another, and whatever other variable you can think of, the test is what it is...done by amateurs!
I believe that stopping is completely dependant upon tire adhesion to the surface conditions, once you reach the limits of adhesion then the tires will lock up. The other rider and I were actually having that discussion before we decided to test our brakes to see who was right, because all he was doing was rattling off marketing mumbo jumbo he had read and was being told, scientifically however it's all based on tire adhesion, and I proved that point to him which shocked him as to the results.
Cheap sidepull brakes had flexy arms, like Walmart type of bike, but once you get into mid to highend that flexiness goes away, and completely negates any differences. I can also guarantee you that if I took my high end older Suntour Superbe side pull single pivot brakes against the guy with the disk brakes the results would be very close to the same. I heard this nonsense for a long time when dual pivots came out that they stopped quicker than single pivots, not true, especially with the high end silky smooth Superbe brakes; and the same is now true with disk brakes. it's all marketing.
However keep in mind, I'm only talking about AL wheels, this information I've been discussing does NOT apply to CF wheels.