First point for consideration as an answer to the original post:
Left to right: .38 special + P 158 grain hardcast keith lead swc; .357 180 grain hardcast lead flat nose; .44 special 255 grain hardcast keith lead swc
.44 special has a notable size advantage. In most cases, however, .357 will achieve greater velocity.
All of the above rounds are penetrating cartridges designed to go through media and come out the other side. They all blow through gel, and they are all traveling over 1000 ft/s. The .44 makes a much bigger hole.
Let's compare energy of the cartridges:
.38 special + p 158 keith at 1050 ft/s: 386
.357 180 lfn at 1300 ft/s: 675
.44 special 255 keith at 1100 ft/s: 566
At the hotter end of the spectrum, .357 retains an energy and momentum advantage, whereas .44 special will almost always be larger and will come close to .357 energy while being much quieter.
However, at the velocity you are likely to see in a snub, the numbers are pretty close together.
140 grain Barnes XPB at 1100 ft/s: 376
200 grain Barnes TAC-XP at 850 ft/s: 320
And it flips when you try other variations:
.38 special +p 158 keith at 1050 ft/s: 386
.44 special 200 grain hardcast wadcutter at 950 ft/s: 400
There are a range of 200 grain .44 special cartridges worth considering, shown here next to the 255 keith
Previously, the Charter Arms Bulldog and the S&W 296 were seen as EDC-worthy options for the caliber. The 296 is shown below, an airweight snub with 5 shot capacity
However, these are downright unpleasant to shoot, and the short barrels limit the slower velocity of the caliber even further.
The S&W 696 solved the weight problem. All steel, external hammer. If this were still in production, it would be a solid competitor to the GP100 version that we are discussing. However, it is quite rare.
If the GP100 can handle the heavier .44 special cartridges, then you might expect energy over 500 for a 255 keith round at around 950 ft/s. This is really quite compelling given the subsonic qualities and comparably low pressure of the round. With moderate loads in both calibers, you are in the 350 to 400 range for energy, and the .44 special weight and size makes you look twice.
Additionally, the novelty of the item is a draw. People have wanted a .44 special GP100 for quite awhile. That is reason enough to make it and even perhaps to own it.
However, the GP100 is pretty darn heavy. Instead of losing 1 bullet by switching to a .44 variant, I would probably just put the Blackhawk on my belt and retain 6 shot capacity