My suspicion is that barrel porting close to the muzzles reduces only that part of the recoil impulse generated by the rocket effect.
That occurs primarily AFTER the projectile exits the muzzles and might therefore have little effect on regulation.

Magna-porting on the other hand acts to counter muzzle rise by directing some of the rocket effect upwards, forcing the muzzles down just before the projectile exits the muzzle, and might therefore be expected to have some effect on regulation.

Of course this is pure speculation on my part, probably worth precisely what you payed for it!

Still a hell of a thing to do to a double rifle!