Now this conversation is getting really interesting!
Let me start by mentioning I've either owned or had several different pistol caliber carbines through my hands over the years. They've all been .357 or .44 Magnum lever action and single-shot rifles though. I'm quite familiar with how much of a velocity boost you can give a 240-grain .429" slug in a 20" Marlin versus a 6" .629. If my memory is good, it was roughly a 400-fps velocity increase. I've never chronographed a set of .357s for comparison, but I imagine similar gains. Oh, and I used to plink steel targets at 200 yards with said Marlin .44 Magnum just because I could. Once you know how much front sight to favor, it isn't a difficult trick at all.
However, I've never had an auto-pistol caliber carbine, but I can see why they wouldn't experience much in the way of gains. There simply isn't enough case capacity to get enough powder to take advantage of the longer barrel lengths.
OK, now to the points about pistol caliber carbine versus a rifle caliber carbine, I have some more thoughts:
1) The PCC isn't a replacement for a full-power rifle. The end. What it does is increase the hit-probability and shootability inside of pistol ranges. While you and I can can hit steel silhouettes from 50 yards with a handgun, most people can't hit anything with a handgun past 15-20
FEET. Remember, this is why the M-1 Carbine was developed.
2) Sometimes a full-power rifle isn't the answer. Steel plates at 5-50 yards are one of those situations. I can't shoot the plates on our family range with 5.56mm and expect them to last very long. But they'll last near indefinitely if subjected to handgun and .22LR power levels.
3) I suspect most people who have a PCC for a long-arm probably have a more powerful long gun available. Just myself, I can put my hands on a 5.56mm carbine, a .30-30, a .308 Winchester or a .30-06 without any trouble.
4) The PCC does have reduced muzzle blast and noise signature compared to a high-pressure bottleneck round. This is important with somebody like perhaps my girlfriend. She's a pretty good shot with a .22LR rifle or pistol, but the extra blast of a centerfire tends to throw her form off. Something in between the .22LR and a 5.56 would be a great step for her to get used to the extra noise.
5) A PCC is cheaper to feed than a full-power rifle. Practice makes proficient.
For the record, I have fired a few true, select-fire sub-guns. An Uzi, an MP-5 and an M1 Thompson. Honestly the 9mms didn't do a whole lot for me at the time. The Thompson was fun, though heavy. To be fair, my favorite select-fire so far was an AR-18 I got to put a magazine through. That got me to consider selling spare organs...
Oddly enough, after all of the reading, thinking and writing, I'm beginning to wonder if maybe an octagon barrel Marlin in .357 Magnum might be kind of fun instead. (Yeah, both is the ultimate solution to this.