From the reveiws that I've read in the gun mags, they seem to reflect the same points...that porting DOES help reduce flip and to a lesser degree felt recoil. The degrees of which are dependent upon the gun, type of porting and ammo. Therefore, nobody can give a standard percent figure for the amount of decrease in recoil; unless they include all of the previous information, which even then is somewhat subjective.
The two only drawbacks, three if you use a real expansion chamber port. That is, an expansion chamber port will add some weight to your carry piece and might affect pointing characteristics/balance. This won't be a problem w/the Magnaport or variations to the "whale blow hole" type of porting: But then again, they won't work nearly as effectively as large expansion chamber ports.
1. Porting can cause gasses to eject out the ports toward the face. This has already been addressed. Only wish to add that this depends upon the direction in which the ports are drilled....Some are canted back, while others are canted forward. Others straight up.
2. Porting increases the loudness to the one holding the gun. Why? Because the explosive gasses are directed closer to the ears, rather than away. I do not know if this matters in a small room where sound reflection is high.
As to the issue of muzzle flash:
Those who have experimented w/the same guns and ammo combinations out of ported and non-ported barrels have said the following. Night vision is pretty much gone after the first shot out of either barrel. It does not matter if it is ported or not. If the flash is enough to cause the pupils to constrict and night vision to be lost w/a comp, it w/do the same w/out a comp. You have muzzle flash from both. The amount of flash is somewhat insignificant considering that recovery time is still going to take a long time in relation to a combative scenareo.
The amount of ambient light available, is a factor, however, if the situation is bright enough to not cause temporary light induced blindness, the extra flash from porting is pretty much ignored if you are used to shooting the gun on a regular basis. It may be noticably more if you are new to the gun, or paying attention to the flash, but eventually it is a moot point as you become accustomed to shooting it. Hope this helps.
robert
------------------
"But now, he that hath a purse, let him take it, and likewise his scrip; and he that hath no sword, let him sell his garment, and buy one." -Jesus Christ (Luke 22:36, see John 3:15-18)