This bears repeating:
A SD round isn't going to do much good if it doesn't feed well in your firearm, you cannot find/afford it, and can't hit with it. Performance measures like penetration, expansion, and retained weight are important considerations but minute.
I have long held something I was told long ago--that if you do not consistently practice with your SD load, it will be a great mystery when at a time of greatest need for surety. Which is why I have always stayed away from the various "super death kill magnum" rounds out there. Like Glazers at $3 per each, or the RIPs at around $6 per each.
As six round or ten round "group" is no real group at all. Tossing down $40 or $60 once a month is a very spendy way to be semi-confident in how your SD ammo "points"--and performs, too. All the wonderammo out there, for all that it is touted as being more sure than first bicycles and sliced bread, is pointless if it jams, fails to feed, ejects perversely, or any of the ills ammo can go through.
OP questioned many thing, and one stood out to me, the question on penetration.
This is a poorly understood thing for being common use.
The "FBI standards" are based on a artificial (if uniform) test medium--ballistics gel.
The FBI asserts that they have, extensively (and if there's anything the Feebs do well, it's research and collate data), compared actual shooting to performance of the same ammo in the testing medium.
By aggregating the data, you can "average out" the variables in armed combat. Confounding things like heavy clothing, defensive postures where arms or legs are raised in protection, all those things that muddy up the calculus of such things. From that data, they converted that into penetration depth in inches in a engineering repeatable medium.
So, the quoted penetration values are just that--how far a given ammo pokes into ballistics gel. (Which despite numerous statement to the contrary does not resemble animal bodies at all--which are not at all uniform and are filled with a great deal of void spaces and the like.) It's about repeatability and uniformity.