Did you realize the M&P 9mm & .45ACP slide and frame are the same size?
Well, almost. My .45 ACP will fit the leather holster for my 9mm like a glove except the .45 barrel and slide is a little longer than the 9mm service pistol so the barrel and slide of the .45 will want to poke out the bottom of the holster.
But the frame length (i.e. dust cover length) is the same. If your holster is molded straight at the muzzle, the .45 may just slide right in, even if it's a kydex holster. But I haven't confirmed this yet since I don't have a kydex holster - yet. I have a Blade-Tech M&P Series Holster for my 9mm on the way and as soon as it gets here, I'll confirm this.
The grip of the .45 is just a bit bigger, but not by much. Of course, since the grip doesn't go in the holster, it doesn't affect holster fit. The Blade-Tech M&P Series Holster is contoured for the dust cover, but straight for the slide and barrel - it just may work, well the slide and barrel will poke out just a bit.