As someone ran on a .mil PSD for a principal who spent a lot of time in harms way for about 5 years, I think this is a piss poor idea and a HUGE waste of resources. Reasons:
1. Size/weight. A pistol caliber SMG is always going to be so large and heavy that it will NOT be carried by the principal as a matter of course. For detail members, the size and weight will dictate it generally being carried in a concealed "bag" of some sort (backpack, sling bag, whatever), meaning it will be GENERALLY available, but not IMMEDIATELY available. In protection, you dance with the girl that brung you - RARELY is there time to "gear up" after things get dicy. We had access to MP5KNs, side folding PDW stocks, and, IIRC, they came out of the armory TWICE in 5 years of CONUS and OCONUS use - because they were just not worth the effort. If the weapon is going to be carried openly, a Mk18 makes INFINITELY better sense.
2. Performance. No matter how "cool" the 9mm SMG is (and I DEARLY loved my MP5...), it's still a large, heavy handgun, shooting handgun rounds very quickly. That means limited armor penetration, limited range, etc. Again, existing solutions in 5.56mm (Mk18 or the dearly departed Mk16 with CQB barrel) offer SIGNIFICANT increases in performance. If we're going to spend the money to develop something "new," let's get something WORTH THE INVESTMENT. Some of the teeny tiny 300BO "PDWs" come to mind...