Since when were we getting M&P 40's? I haven't heard anything. I have read that the USMC recently placed an order for a bunch of 1911's. I have heard nothing of a new pistol for the Army.
Pistols aren't even highly used by the Army except in MP units. Other than that only officers and high ranking NCO's carry pistols. SOCOM forces typically carry non-standard weapons but the M9 is still the standard issue item. I have read that the SEALS are now using a SIG 9mm, but still a 9mm.
I personally don't like the .40. For me its either 9mm or .45ACP in a pistol. 9mm for ability to typically hold more ammo and .45 for its added punch. That said, I am fully confident in my FNP-9 to kill a person and quickly. I carry Federal Hydro-shocks in it for that purpose. When it comes down to it, IMHO the type of bullet is more important than the caliber. Regardless of the caliber handgun the Army may or may not go to its still going to be firing FMJ's and therefore will have a compromised ability to kill on impact.
I have only shot a few .40's but I've not liked any of them, and quite frankly I don't see how a bullet .045" bigger in diamet is going to be so superior. Its firing slower than the 9mm and has barely any extra mass. If you want to step up to a "big boy" caliber than just man up and get a .45