Humm, good question!
There is no doubt in my mind that the parallel-to-bore axis Mag-na-ports on my 6” S&W Model 27 definitely improve my shot-to-shot times and allow me to hold onto a better sight picture. This has, also, been the consensus of opinion among other shooters who have occasionally used this gun in competition. Frankly I would prefer porting on every hunting gun I own – porting belongs on hunting weapons.
The necessity, even the relevance, of porting on combat weapons is more open to question. I’ve, also, got perpendicular-to-bore axis ports on two of my 6” barreled Colt 1911’s. Porting in this instance works very well. Because the ports are on the last inch of barrel beyond the slide, the sights are not affected; and there is minimal flash visible upon firing. (At least that I’m able to see.) Even with, ‘full house’ loads, I do not find the recoil to be excessive, nor the perceived flash to slow me down. Quite the opposite, actually. Both of these long barreled 1911’s put the rounds exactly where the muzzle is pointing with minimal sight picture disturbance.
I used to own and frequently use an S&W Model 19 with a ported 2 1/2” barrel. This weapon had lots of flash to it, and always blackened the sights after, about, a dozen rounds. Personally, I don’t buy into the idea that you can’t use a ported gun effectively from, ‘retention’. Sure you can! This is, however, a DS situation in which you shouldn’t be watching the muzzle, anyway; and, if you’re already in this much, ‘doo-doo’, who cares about powder burns on your clothing? I would think that blood and guts splatter would be more of a concern!
This being said, I would NOT buy any of the modern semi-automatic designs with cuts in both the slide and barrel for, about, the same reasons that I, also, wouldn’t use any 44 magnum pistol in any sort of combat scenario. (As a matter of fact the last time I attempted to use a 44 magnum in IDPA-style shooting, my times positively stunk! I was, still, able to shoot straight enough; but, there was no way I could keep up with the other shooters on a shot-for-shot basis.) Pistols with ported slides and barrels are DEFINITELY NOT ideal combat weapons. The muzzle flip does go down; but the sight picture is significantly degraded by the excessive: flash, noise, and subsequent distraction. You're going to lose time; and, in low light, this brilliant flash will quickly dilate your pupils and dangerously reduce your night vision.
IME, ported weapons are much like people: You shouldn’t rule them out, categorically. Ported weapons have their uses; and you have to consider each one as you find it. Some porting designs and systems work very well in given applications while others work not particularly well at anything. Certain types of porting work better than others, too.
As a general rule you would be wise to leave ports off combat guns.