I do most of my squirrel hunting with a 12fpe .177 HW77K. I shoot hard Crosman Premiers out of it. Same as I use for field target.
At 35 yards, it will leave the pellet hanging just under the skin on a big boar fox squirrel on a heart lung shot. That's hitting at about 8fpe. They fall dead, right there. The pellet is not mushroomed, but it is deformed.
To me, I don't see it getting any better than that. That's all energy dumped, as much penetration as possible.
I've been doing it this way for a couple decades now, shot all kinds of pellets, and can say that in my opinion, on a good shot it doesn't make a poops worth of difference what the pellet is. And on a bad shot, an expanding pellet won't save it. It's just a bad shot.
Domed pellets almost always shoot flatter from their better BC than flat nosed, hollowpoint, or polymags. This makes it easier to hit what you are aiming at.
And really at the end of the day, hitting where you are aiming is what makes clean kills.