Not an uncommon question ... to hear from .45 shooters.
Look how long it took Glock to offer a Practical/Tactical 5" .45 model (new G41).
Now that they do, though, perhaps S&W will closely watch that part of the market and see if they think it's worth cutting into their regualr M&P production.
That's presuming they haven't already got a 5" Pro M&P 45 on the back burner, waiting for the time when they can produce enough of them to introduce them. Dunno.
I used to think I'd want a 5" M&P 45.
Now I'd shrug and let it go by. I'd probably feel differently if I was involved in a competitive venue, though.
The barrel on a FS .45 is already longer than a FS 9/40. (4.5" vs 4.25")
The FS .45 is one of the most accurate guns I've ever experienced out of the box. I can't imagine another 1/2" of barrel really making a significant improvement.