A 180g bullet loaded to about 700 fps gives equivalent recoil to a 115g 9mm bullet close to 1100 fps. On my M&P 40/9, I've run both loads (40 and 9) and they really do feel the same so that is a good comparison if you want to know what a 9mm load would feel like in a 40 gun.
That being said, the M&P 40 slide is slightly heavier than the M&P 9 slide and 9mm loads will not eject with the same authority out of this gun compared to a real M&P 9. While the 9 and light 40 loads do cycle reliably and lock the slide back, the cases fly up only 1/2 to 1 foot up and often land on my arms, head and forehead. I need at least 1100 fps out of a 120g bullet for the cases to eject with authority in this gun. I don't care about getting max velocity for any ego sake, just enough to not be annoyed by having hot cases land on me while shooting. A reduced recoil spring would help, but then you have to deal with a weaker slide closure so I don't want to try that.
IMO, no need for any porting on a 9mm barrel, the load has such low recoil and flip already and having a firework show in your sights won't help. Just a plain 9mm barrel with 9mm mags will work fine.
