Jon,
My observation has been that many of the departments running metal framed SIGs are simply switching over to the striker option from the same manufacturer. Most of the departments running other guns are still running them.
What departments do we know of are actually selecting the P320 because of the unique features of the P320 that are not present in other striker fired polymer options?
What's the percentage of departments switching over to the P320 who did not previously carry SIG metal framed DA guns?
My suspicion is that PDs are saying, "It's time for metal framed DA guns to go away", and so they're swapping to the P320 out of convenience......not really due to the MHS program, any unique attributes that the P320 offers over Glocks, M&Ps, or whatever. Maybe I'm too cynical, but I highly doubt that most PDs adopting them are doing so because of the relatively inexpensive grip modules affecting index. They want a cheap striker fired plastic gun, and it's an easy solution from a supplier and support network they're already working in. It could be a rebadged SD9, and most of them would probably still buy it.