If the factory puts in a bobbed hammer, and the factory says its all right, then the factory stands behind it, and should the gun have trouble, then the factory will fix it. I'm good with that.
If you bob the hammer, then its your issue, if there is on. And if the gun goes on the market, as a buyer, all I know is someone bobbed the hammer, and that someone isn't around to call and fix it if there is an issue down the road.
I understand how guns work pretty well, but I'm not set up to make engineering calculations like the factory is. If they decide the stock spring is still the right one with a bobbed hammer, I won't argue with that. I'm counting on them to have made the calculations and to produce a sound, reliable product. That's what I'm paying for. And I'm also counting on then to stand behind what they sell. That's another thing I'm paying for. SO, from the factory, I', ok with that.
Used, done by who knows, and also with no idea what, if anything else was done, that, I'm really leary about. Some folks will bob the hammer AND put in light springs, and sell if it has problems. USED, its buyer beware, and I am...I think you ought to be too.
All else being equal (and it almost never is) bigger bullets tend to work better.