That's not how I approach changes.
The main thing to understand is that I don't have that kind of control over outcomes. We write code and set variables. We throw a mess of problems at you and give you a set of tools to address them. Then I watch feedback to see how campaigns are going. (I'll add players frequently surprise me with their abilities to use tools in ways I hadn't considered.)
When I make changes that affect all difficulties, it's generally to keep certain strategies from becoming dominant. Dominant strategies ultimately cut off options and render content we've worked on useless. Sometimes those changes means buffs to XCOM, sometimes it means nerfs. (Yes, the nerfs get far more attention when announced). Then I watch feedback to see how it's playing out, and part of the purpose of my post above was to lay out the sort of feedback that can really help me make changes.
The mod is an iterative process, and a collaborative one with feedback from the player base playing a huge role in adjustments we make. And that feedback I'm extremely grateful for, which is something I really should have made clear in my original post.
As for impossible, as I said I approach it in a somewhat different way. I don't know -- can't know -- what portion of the player base can beat any given iteration of it, so I really don't think about that when making changes. I throw it out there, see feedback of how it is being beaten, and on occasion tweak it up in response. Sometimes feedback from impossible players is applicable for all difficulties; sometimes it's not. I don't think a damage bonus to the aliens renders it unbeatable, just harder. (I can think of silly ways to make it truly impossible, but you'll notice me not employing them.)
What I can't really control is people who automatically select the highest difficulty level thinking it's going to meet their definition of fair in spite of the warning we put in there. It's really for people who want the best test we can throw at them and don't mind losing over and over again, and players who are really, really good at the game. Those folks deserve a difficulty to play on in my view. So do casual players, which is why we've put in Cinematic mode (sorry about the bug in 15, happened when I was cutting and pasting to reorganize the ini) and lots of other tunable options in the ini.