Years ago, I had 2 .357's ported, a Python & an S&W L frame, both 4" barrels. I can't really say how much it helped w/o having an unported example to compare side-by-side. One thing I have noticed, when I shoot cast bullets, I get lead build-up on the top of the barrel ahead of the ports, which leads me to believe there is some gas cutting of the bullet base. Whether this affects accuracy to any measurable degree, I don't know. I may not be a good enough shot to tell the difference. Maybe I should load up some identical loads but with poly-coated bullets & experiment. But it will have to wait for warmer weather.
Also I have a Hellcat RPD with the comp on the muzzle. Again, I don't know how much difference it makes. Come spring, I'll try shooting it with & without the comp attached & see if it makes a noticeable (to me) difference.
As far as reduced split times, I can see it making a difference for competitors, but in the real world, as in self defense, I don't see dropping .15 seconds making any practical difference.