John, recessing the muzzle end of the sleeve to conceal the barrel flange was only a cosmetic improvement, the Model 66-8’s yoke and cylinder were redesigned to make room for the rear of the barrel to be thicker. The rear end of the 66-7’s barrel all the way around is about the thickness of the thinnest spot in older one piece K frame .357 barrels. However, despite the thinness, the 66-7 got rid of the stress inducing flat under the forcing cone that caused a very small percentage of one piece .357 barrels to crack. So, the 66-8 barrel is stronger, but only on the other end.

to lower production costs to keep the guns at a certain price point

I've heard the only reason S&W went with the two barrel design is because it's cheaper to install.