Magna porting DOES reduce muzzle jump, which makes for a faster second shot, however in a bolt action rifle, that is a moot point as the shooter has to move the rifle to work the bolt. Magna port advertises a 15% reduction in felt recoil with Magna porting vs a 40-50% reduction with a muzzle brake. I would not have a bolt action rifle Magna ported, but if a previous owner had a relatively light kicking rifle Magna ported, it would not stop me from buying it, IF the price was right.
I have what I consider, two heavy kicking rifles, a .300 Weatherby and a .375 Rem Ultra Mag. I had muzzle brakes installed on both of these rifles, and I would not now shoot either one without their brake.
Magna porting would not have an appreciable effect on a bullets velocity, but it will increase the muzzle blast and sound waves to the shooter and to others close by. Sound waves radiate out from any hole in the barrel. When the only hole in the barrel is at its end, then all of the sound waves emit there and radiate AWAY from the shooter. If there are any holes or ports in the sides of the barrel, SOME of the sound waves will emit from those holes and some of that sound will radiate back to the shooter, making the report louder to the shooter.