Adding a five inch S&W Pro slide and barrel to a standard frame is no problem. I did it and it went a thousand rounds or so with no problems. S&W lightens the five inch slide so that it performs just like the four inch. I did it the "hard way"; I bought a Pro model used in MA. The reason I switched the Pro slide and barrel was that the four inch frame had been stippled and offered a superior grip.
S&W does not seem to be intersted in selling these parts in MA unless you are a "special friend". The only way to do it is to buy the whole gun used in MA. The real kick in the butt was that the front sight fell off after a few hundred rounds. I called S&W customer service and they told me I might as well fix it myself! I was told that a bunch of slides left the factory with oversized front sight dovetails that wouldn't hold the factory sight. The tech told me that the only thing they could do was disrupt the metal on the base of the front sight with a punch for an interference fit and that I might as well do it myself. I replied that this was a "bush league hacker" approach to the problem and asked why they didn't buy the appropriate front sight from Brownells? The tech had no answer so I gave up and bought the front sight from Brownells. The gun is now fine, problem solved. Unfortunately S&W customer service is starting to suck big time.