I love learning about new cartridges, but often it seems the new fodder coming out only overlaps (and sometimes replicates) existing cartridges.
It seems to me, for the most part, this is one of those cartridges. In terms of ballistics, the .460 Rowland seems to do everything the .450 SMC does and more.
I'll give some examples below but from what I can figure, it boils down to the .460 Rowland being the same overall length as the .450 SMC yet operates as a max pressure of 40,000 PSI whereas the .450 SMC tops out at 32,000 PSI.
The velocities listed above in the OP for the .450 SMC are from a 16" barrel. From typical handgun length barrels, here are some numbers to kick around (approx 5" barrels)
185gr JHP
.450 SMC (Double Tap) 1350 FPS 749 ft/lbs
.460 Rowland (Underwood) 1575 FPS 1019 ft/lbs
230gr JHP
.450 SMC (Double Tap) 1130 FPS 652 ft/lbs
.460 Rowland (Underwood) 1400 FPS 1001 ft/lbs
255gr Hardcast
.450 SMC (Double Tap) 1030 FPS 601 ft/lbs
.460 Rowland (Underwood) 1300 FPS 957 ft/lbs
Considering the ballistics above, I simply don't see a niche for the .450 SMC. If the .460 Rowland is viewed as overpowered, a reloader can address that with ease to replicate the milder ballistics of the .450 SMC. It does necessitate the use of .460 Rowland brass (and corresponding barrel), but this also prevents the user from sticking the .460 in his unconverted 1911 and soiling his pants or worse.
The only concern for reloaders of the .460 Rowland is the issue of primer flow. However, being the rounds listed above are all factory loads, this shouldn't pose a problem.
A few years ago, I converted a Springfield XDm 5.25 .45acp to .460 Rowland. A replacement barrel/compensator and recoil spring was all that was needed.
I purchased a couple hundred rounds of "Johnny's 240gr Magnum JHP" off Rowland's official website for $60-ish/50. It's a 240gr JHP (designed for high velocity) at 1300 FPS for 901 Ft/lbs. Recoil is more substantial than the .45acp but nothing like one would expect with such ballistics. I don't think it will beat up a modern-manufactured handgun. Unfortunately, the muzzle blast is obnoxious for an automatic. Firing at an indoor range is nearly as disruptive to others as my AR pistol.
I can honestly say I've fired this pistol less than 50 times as I still have a few round left from the first box. I used to carry this for trail use since 13+1 rounds of something within .44 mag ballistic territory sounded swell. And it was... but I carry the RIA 10mm for such use now simply as I have more rounds through it (I reload for the 10mm, not the .460 Rowland... yet).
Images of cartridges below pulled from AR15.com