Did searching of YouTube for video of MP9 hit probability.

This is the best I found; max target distance only 10 yards.  

YouTube

Testing the Ultra-Compact, Full-Auto B&T MP9-N Subgun

With a 5-inch barrel and a collapsible length of just 11.9 inches, the nimble, compact Brügger & Thomet (B&T) MP9-N reinvents the class of 9mm submachine gun...

stancrist said:

Less recoil would help, but the flatter trajectory won't make a bit of difference for most soldiers

Its hard to point to with like a ballistics chart, but I've found my FN FsN shockingly easy to fire off hand at 100yd with just iron sights - much easier then a 9mm. 

I once shot a beer bottle at 80yd with mine, and was able to hit a gallon milk jug at 150yd within 3rds. (Alas I dont think my eyes would be able to see a beer bottle at 80yd any more...)

Its really as close as it comes to a laser pistol.

With the latest variant equipped with a RDS (or a proper RDS and stocked PDW) hit probability will likely be noticeably better then 9mm. 

The reduced Time of Flight may also help increase chances of a hit for moving or temporarily exposed targets. 

gatnerd said:

       stancrist said: Less recoil would help, but the flatter trajectory won't make a bit of difference for most soldiers

Its hard to point to with like a ballistics chart, but I've found my FN FsN shockingly easy to fire off hand at 100yd with just iron sights - much easier then a 9mm.  I once shot a beer bottle at 80yd with mine, and was able to hit a gallon milk jug at 150yd within 3rds.  Its really as close as it comes to a laser pistol.

I stand corrected.  

BTW any idea as to why the high dispersion? Recoil?

Maybe, but I'm not certain, as I've never shot a 5.7 pistol.  Since gatnerd has firsthand experience with both calibers, perhaps he can offer better input.

Sorry, I meant the MP9

Yes, the dispersion of the MP9 might be due to recoil. 

Note the tester's comments about the semi-auto TP9:

@ 11:47-12:49 https://youtu.be/GXR5zq1Gh_M?t=707

@ 16:49-17:05 https://youtu.be/GXR5zq1Gh_M?t=1009 

gatnerd said:

As I've said numerous times, and echoing Tony's comments, where the PDW makes the most sense is for the 'HE lobber.' A infantry guy whose primary job is employing man portable HE weapons (40mm smart grenades, 84mm CG, ATGMs, MANPADS, etc.)

As you know, the Marines do not share that view.  The new 15-man squad has every member equipped with the M27 rifle, including the guy who is tasked with toting the 84mm CG.

Interestingly, I happened to watch a USMC video (filmed late last month, I think) of a combat readiness evaluation in which the CG teams are armed with the M4 carbine, not the M27.

1:51-1:53  https://youtu.be/46aZ2zzUgjs?t=110

6:23-6:55  https://youtu.be/46aZ2zzUgjs?t=383

I suspect the Marines have realized even they have only so many supermen who can carry a CG and an M27. The little less weight of an M4 would be nice. And since M4 and M27 use the same mags, wouldn't  be too much of a problem using both weapons.

That seems logical.