The SP101 eats out of J-Frame S&W speedloaders, so we generally call it a "J-class" critter...but it's stronger, at least as strong as a S&W K-frame and possibly stronger.
Phrase it this way: buy a 3" SP101, and you've got a good street defense piece that in a pinch can take real monster 180grain hardcast hunting loads for "woods defense". It's the smallest gun available that I'd load such "heavies" in - recoil will be serious, but not impossible. If I was *planning* on shooting/carrying those now and again, a bigger gun would be in order, but the SP101 can be pressed into that duty in a pinch.
I don't think I'd load those in a S&W or Taurus 5-shot stainless snub. I know for damnsure I wouldn't load 'em in a Ti or Scandium snubby, not for any amount of money.
This is why we say the SP101 is really "it's own thing". It can be a GOOD thing, or heavier than what you need, it's up to you. My opinion is, in an IWB carry it's not at all too heavy, and you gain controllability and toughness. If you want to carry in a pocket, you'll consider it too heavy.