So, I went to the Hornady site 'cause they have a simple, easy to use on-line calculator and decided - arbitrarily - that a 124 grain 9mm at 25 yds would be my benchmark for energy imparted into the target.
I am going to round the numbers because they are all very rough estimates anyway but at 25 yds a 124 gr 9mm traveling at around 1050 or a little faster (subsonic) imparts around 300 ft-lbs into the target.
I then decided to be conservative and assume that for the 5.56 we would loose 50 fps for every inch of bbl decrease. So for a 16" bbl we can deliver that same approx 300 ft-lbs of energy using a 55 gr bullet at somewhere around 475 yds and the bullet remains in the supersonic range.
For the 7.5" bbl, again using the same 50 fps loss per inch and the same 55 gr bullet we deliver that 300 ft-lbs at around 325 yds and again, remain supersonic.
As I stated , these are very inexact numbers and suitable only to provide a relative comparison. So you just need to decide if the loss in performance is justified given other relative advantages of the very short barrel. For my purposes, the pistol provides an excellent CQB solution, but I am also not a very big guy. Some of you could probably hump a 12 lb, 20" bbl .308 as easily as I can handle the AR pistol
and should be abolished along with I dare say the ATF itself. 
Just obtained a 2nd pistol, am SBR'ing the first before the 13 July deadline. Now I need more ammo!!!! 