I had a 31 for a little while. Ended up getting Lone Wolf 9mm barrel for it and then selling it a short time later.
My main concern with it was the battering/peening the underside of the slide was getting from impacting the locking block. I was told it was common with the .40s, but the couple of .40's looked at didnt appear to have it to the extent that my 31 did. It also never appeared to slow down, as I was also told it would. My 17's and 26's shooting more rounds +P+ 9mm than the 31 had through it, show no signs of any peening.
So look at it this way, any run of the mill 357 sig load is like getting the highest performance subgun +p++ 9mm ever loaded plus another 100fps sprinkled on top just for good measure.
I was quite enamored with the 357SIG for a number of years, and spent my share of time arguing its merits. Since no one seemed to have "actual" facts in any of the discussions, I emailed Speer awhile back and asked them directly. This was their response.....
" The 9mm is a 35,000 psi, +P is 38,500 psi and +P+ is 40,000 psi. The 357 SIG is a 40,000 psi. Bullets of the same weight will approximate the same velocities in SIG and +P+. The difference is gun construction, all 9mm's will not handle +P+. All of the 357 SIG's are made to handle the pressures for the caliber.
Shoot Straight!
Coy Getman
CCI/SPEER Technical Service
2299 Snake River Ave.
Lewiston, ID 83501
(800) 627-3640 ext. #5351 (pound key must be used)"
Basically what I got from that was, +P+ 9mm and 357SIG shooting bullets of similar weight, will perform similarly. I can tell you from shooting a 17 loaded with +P+ and a 31 side by side, they shoot and react the same. If you were handed one and were asked what it was after firing it, I seriously doubt you could.
Interestingly enough, their comment about guns made for 357SIG handling the round, and some 9mms not handling +P+ seemed to be reversed in my case. My Glocks that have a good bit of +P+ through them, and more than my 357 did, barely show finsh wear on the slide, where the 31 was badly peened.