Your first question was about the quality of work by Magnaport - so first - they're as good as anyone in the market, and better than most. If I were a fan of porting in Alaskans, they would be my go-to company to do it. But as you'll see below, I'm not a fan of porting in Alaskans.
Your second question asked if magnaporting would render the 454C useless against large game. Remember - a magnaported Alaskan in .454C will still have greater muzzle velocity, meaning greater energy and momentum too, than a long barreled 44magnum or 45colt, either of which would readily and rapidly kill even the largest of bears. No, doing so will not make it useless against large game.
The question you didn't ask, but what I feel is the linchpin question for this project: "Is magnaporting an Alaskan worth doing?" To which, my answer is no. While magnaporting won't cripple the 454C, it will, however, elicit more visible flash and more concussive blast to the shooter as well. It'll help take a bit of the bite out of the top of your hand, but in my experience, not enough for me to spend money on it. The recoil taming benefit is real and noticeable, but it's not even as noticeable as a reduced power load - and frankly, reduced power loads are readily available even to a non-reloader, as many factory loads operate at 50kpsi instead of the SAAMI MAP 65kpsi, and "45Colt +P" loads are readily available as well. So for me - given the trade off of lost MV to get reduced recoil - a guy can do the same (or more) by changing the powder charge in the case instead of drilling holes in the barrel.