I picked up an M&P40FS a few months ago and really like it. I've shot 9mm, .40 and .45's in the past. All seemed just fine to me as far as recoil goes.
If you already have a M&P9, why don't you just get a .40 barrel and magazine and try them out in your frame? The 9, 357 Sig and 40S&W frames...