IMO, using FMJ's and LRN's is like stepping down in power and/or caliber. Using a LRN .357 as an example.............. .Is it now on par with a .38sp? How bout 9mm? .380? .32 H&R? I don't think any of will ever have enough info to make the call and know for sure how much worse it is. But one thing for sure, you do step down in effectiveness but retain ANY and ALL negatives that come with more recoil and bigger guns.
Actually in my experience commercial LRN bullets seem to be pretty soft. They don't lead barrels because they're not being pushed very hard.
My experience is the exact opposite. You can order lead bullets in many different hardness's but most off the shelf stuff you find at gun stores, box stores and gun shows are hard. Very hard and too hard in fact. Check the hardness of many of the common cast stuff and you'll see why it's referred too as "hard cast". Also, lead that is the right hardness and sized right can be pushed plenty fast. In many cases as fast as jacketed bullets. But most of the "hard cast" stuff (even sized right) will tend to lead a barrel faster than many softer lead alloys. Lead will indeed mushroom/deform if it's the right alloy. As a matter of fact if your lead isn't deforming some it's probably too hard.