If you're shooting up the trees, then fluflus with rubber blunts are the only way to go. Anything else will get stuck up there.
I have killed many squirrels with steel blunts, but like you, haven't been thrilled with their performance.
I currently use a field point with an adder behind it for small game. I haven't shot a squirrel with it yet, but I have shot a bird and a roadkill possum with them. On the bird it was an instant kill, complete pass thru, dead before it hit the ground. In both tests it achieved enough penetration to kill, and the adder tore a nasty hole going in. An added bonus to them is that if you don't get a pass thru then the adder acts like a barb and keeps the arrow in the critter, makes it a little harder for them to run up a tree.
Just my 2 cents.