I can't believe S&W told you that only a small amount of +P should be used in a Model 15?!?! That Model 15 will handle all the +P you care to feed it......
S&W maintains that any "model number" revolver can handle any +P .38 Special, but I think they use this as a "CYA" cutoff. I see no reason why, for example, a S&W M&P .38 made in 1956 can't take .38 +P but a Model 10 made in 1957 can.......
The early S&W M&P's, made approx. 1920 and prior, did not have heat treated cylinders.
The fact is, ammo has been downloaded over the years due to lawyer CYA stuff, so I would wager that a so-called .38 Special "+P" round made today is probably only a little hotter, if at all, than the standard .38 Special made in the 1950's.
I had some Super Vel 110 gr. HP .38 Special, from the 1960's (before Super Vel was bought by another company) and this stuff is hot, more like a light .357. It's not even branded as "+P", it's just .38 Special. My gun dealer had some old boxes of it, so I bought 2.......I saved one full box for my .38 ammo collection and shot up the other box, except for 6 rounds that I use for CC occasionally. Since it's HOT, and uses a 110 grain bullet it's horrible for forcing cones, so I wouldn't shoot too much of it.
This stuff is gold if you can find it, but I don't like to trust ammo this old for life and death scenarios........it's more collectible and "vintage" now. I do want to try some of the .357 if I can find it, I bet that stuff is spicy
I love the "Police Only" mark on the box, I collect and own a LOT of ex-police revolvers, so this ammo would fit great into my collection.
My point is, factory ammo is no where near this hot these days, so the whole designation of ".38 Special +P" more or less means ".38 that's just a little hotter", but many people think it means .38 that's much more powerful.
My dealer had a box of the .38 +P+, I would imagine it to probably be on par with the Super Vel .38. I think I fired some .38 +P+ years back, through my S&W 67-1 but without a chronograph, all I could tell is that it seemed pretty hot but not .357 power levels.