Yes, the lighter .357 M&P J's are a handful when loaded with full-power Magnum ammunition. I've known large, strong guys who didn't want to finish shooting a cylinder load of Magnum rounds after firing 1-2 rounds.
I tried an assortment of Magnum loads in my first M&P 340. I found some which exhibited bullet pull (jumping crimps) when tested as recommended in the manual, and some which didn't.
While I used Magnum ammunition for qualification the first time, and demonstrated I can make fast, accurate hits shooting "doubles" & "triples" out to 10-11 yards, I typically don't carry Magnum ammunition, not do I routinely practice with it.
I usually use +P loads for practice/quals (sometimes standard pressure, depending on availability), and carry the same +P loads. At present I carry either Winchester's new 130gr +P RA38B (same thing as the 20-rd commercial box PDX1, but it comes in a 50-rd Ranger box) or Speer's 135gr +P GDHP. Shooting 50 or more rounds of either starts to take a toll on your trigger finger, though (pounding against the trigger guard under recoil).
I've also carried & used Remington's 125gr BJHP +P and their 158gr LHP +P upon occasion.
I look at my pair of M&P 340's (1 w/lock & 1 w/o) as sort of "improved" 642's (I have a pair of 642-1's). Stronger scandium aluminum frames and a better sight (XS standard size dot night sight & a nicely shaped rear U-notch). The use of an aluminum yoke and sleeved barrel makes it a bit lighter than my 642's, too.
The fact that my M&P 340's came chambered in Magnum was more of an unneeded "extra" for my desires. I'd have been satisfied with it coming in just .38 S&W Special +P (like the LAPD SKU model).
If I want to shoot Magnum loads I'll get the SP101 DAO out of the safe.
I've fired the even lighter PD Magnum J's, but I don't care for the titanium cylinder.
Really light in a pocket holster, though.
Personally, I think the ultra light Magnum J's are really pushing the practical boundaries of the human/gun/ammunition interface.
I've met a gun who was proud of the fact that he'd fired several hundred Magnum rounds through his PD, and he was anxiously planning to order a M&P and exclusively use Magnum loads. Good for him. He's young, large and strong ... and apparently enjoys the experience.
Now, I grew up learning to shoot Magnum handloads through Ruger revolvers, and my idea of a pleasant afternoon was shooting a few hundred stout Magnum handloads through my Rugers. I still find shooting a 2 1/2" M66 with assorted full-power Magnum loads to be a relaxing range experience, and expect it to shoot accurately out to 75yds.
I don't, however, "enjoy" shooting 125gr, 130gr, 145gr, 158gr & 180gr Magnum loads through the M&P 340.
Besides, owning and using more than half a dozen J-frames, I've found I can wring great accuracy out of them, along with good controllability & recoil management for rapid shot strings, using the +P loads I've found to work well in
my J's. Using the +P loads gives me a slight edge during quals & practice drills when it comes to recovery & controllability compared to using Magnum loads.
I don't make blanket recommendations when it comes to guns & ammunition, though. Not for other folks. I prefer to have them try whatever gun/caliber/ammo choice they're considering out on the firing line ... and let them decide for themselves.
Try one and see what
you think, and decide for yourself.
You might also consider shooting one of the Magnum-rated steel J's, too, and consider the weight against both carry role and shooting experience.
Just some thoughts.