I had to take a hammer to my subframe in a couple places. The steel is actually pretty soft. But then "soft" in steel isn't really a worry unless we're talking about springs or cutting tools. Suffice to say the subframe steel is "Strong enough" in that it has sufficient resistance to plastic deformation for it's intended purpose and is malleable enough not to crack/fracture under the standard use reciprocating loads. Being technically "hard" in this instance would probably be a bad thing.
As for damage, I'm not worried about your subframe. If it bends too far, pull the bushing and hammer it back to shape: Experiment over.
I am worried about your diff cover. Like you, I thought that simply adding a fourth pickup to distribute loads would generally lower stress on all points. Unfortunately my first attempt had too much play in it which resulted in a broken diff cover. Frankly, I knew better but was lazy. With the welded in sleeve I've eliminated the possibility of the same failure. In essence, by installing the fourth point carelessly I had introduced a recurring impact load to the system. I added a deformation dynamic to my subframe that exploited a failure point for the diff cover.
Now, those suckers are particularly weak anyway, and I've seen more than one in stock configuration fail because of worn(loose) bushings. In general, aluminum work hardens more than steel and getting more flex from the threaded bolt hole twisting back and forth from an unsecured bushing(or worse, being pounded by a reciprocating hammer loose bushing) is plenty more than enough to eventually split open the threaded hole. It could, as in the case of my buddy Anthony, snap open a hole in the diff cover and puke gear oil all over the place on interstate 15 at 2am. He drove a relatively stock 2.0.
Beating a dead horse here, but seriously, as long as there's no play it should be fine, but as soon as you hear any weird clunking or anything from the rear end please check it out. Maybe inspect it working on a lift once a year or so. ...From a guy who cares and is driving on his third diff cover
As for the '13+ 3.8 supposedly the stock HP is 348 SAE. I don't think the 400hp theoretical limit is a real thing, but it's definitely worth being cautious.