I have been shooting 45 Super for close to two decades now. The M&P would not be one of the firearms that pops into my mind as a candidate for conversion. Personally, I do not have experience with the M&P45 and that may be the reason. Also the USP family makes such a good platform for the 45 Super cartridge that I have not gone looking for another polymer solution.
Have you checked to see if the chamber is fully supported? If not you will need a new barrel as well.
I am sure that you already know to contact Wolff for springs.