Plastic pistol has nothing to do with it. In the case of the M&P the stainless chassis is embedded, or rather cast, into the polymer portion of the frame. The slide and barrel are steel. +P has cracked plenty of steel and aluminum frames anyway.
I'm a big slow bullet fan that penetrates and crushes bone and gets into organs. I also like a regular JHP 230 grain .45 round because it has the same recoil impulse and same point of aim, point of impact, as a practice hardball round.
If you like .45 in +P more power to you. ;-) I carried +P 200 grain for a short time in the past but have returned to 230 grain hollow-points.
Personally I would not use any .45 against a bear. Minimum of 10mm for a semi-automatic but better to have a big honkin' revolver round.