I was thinking the same thing John, how would manga-porting affect the regulation of a double?
Most doubles regulate within 100 fps or so of there desired speed, I simply do not know much about muzzle brakes or porting of barrels - except they divert enough gas to make a fair difference in felt recoil!
But do they in the process drop that much velocity in the last 2 inches or so, if then perhaps like chopping of 2 inches of barrel - perhaps 50 fps maybe?
The question then begs, exactly whereabouts is the regulation of this rifle (hopefully on the faster side & slightly crossing - if I've got that part right) - otherwise if it is on the 'slower' part then it could literally deregulate itself by going too slow?

Still it is a shame to see a double 'defaced' like that - but hey it's the owner's rifle he can do as he/she pleases!
Certainly not my cup of tea to do so that's for sure.