As the next generation of these guns, the 640-1's, were .357 Magnums I don't see any problem with a limited use of +P+. Mine usually gets carried with +P though and mostly fired with reloads for practice.

I lobbied Smith & Wesson back in the early 80's to start building these. Wrote letters, made phone calls, then kept my promise and bought the first one I could lay my hands on.

Mine has taken over duties protecting the kitchen, the most important room in the house. Since I got my M&P340 the 640 just doesn't get out much anymore.