Lots of testing done here.
I have a 3" bbl Smith 686+, and a 4" 686. And I have done a lot of real world chronographing with the two side-by-side with the same ammo.
Full throttle ammo using lighter bullets (125gn w/Power Pistol) or slower powders (158gn w/2400), the difference is about 90-95 f/s. Slightly "de-tuned" ammo using heavier bullets and intermediate speed powders (158gn w/HS-6), the difference drops to about 70 f/s. Faster propellants realize less difference: 125gn w/AA2 - 20f/s; 158gn w/AA5 = 40f/s.
Also of note: I have a 8-3/8" bbl 686 too, and have run the same tests. Usually, there is no velocity gain to speak of - but a few f/s over the 4". I strongly suspect the barrel/cylinder gap produces rapidly diminishing returns. I don't have a 6" 686, but I suppose that's probably the longest barrel that does much good in all but novelty applications (super light bullets w/super slow powders - that sort of thing).