I don't think the logic is sound. Okay, enabling good hits is obviously critical, and the argument for 9mm seems to be that it does that via lower recoil and greater capacity. But a good hit requires more than just good shot placement, it also requires that the bullet continues on the path that shot placement started it on.
Momentum is the key here. Among other things, it's a measure of how difficult it is to change the path of a moving object. A 9mm might nip at the heels of .40 in terms of energy or expansion, but it lags behind substantially in momentum. Comparing 9mm vs .40 HST, using the numbers from Federal's website, the 9mm comes in at 78% of the momentum of the .40.
What that means is, when hitting anything hard - bone, auto glass, auto door - a 9mm is likely to deflect more, which can result in a poor hit (or no hit), even with good shot placement. That, to me, certainly seems more important than having 17 rounds instead of 15, which has no benefit at all until 15 rounds have already been fired.
So I guess the remaining argument is that officers shoot 9mm substantially better than .40. And that, to me, seems like a training issue, making the switch to 9mm a bandaid. If it isn't driven solely by ammo cost, which I suspect it is.