I have split as big as 28". That is so big you have to roll it (cause you can't pick it up). IF it is straight grained, the size of the tree doesn't matter for splitting.
I got the X27 as a Christmas present. I have been splitting all my life by hand, and I will NEVER be without it again. I can split twice as long as I did with my maul without getting tired, and it takes the same number of swings as with the maul.
That being said, I have an 8lb maul, and it does work better on the really stringy stuff.
If my Fiskars sticks a lot, I break out the maul. The maul will not stick, it will either bounce off, or it will blow through.
With nice straight grained wood, I can split faster than a hydraulic. But the first time you get a really knotty, or really stringy tree.......the guy with the hydro will be laughing at you while you curse every piece you pull down to split.
As for wedges.....They are for the really awful stuff. If you have a very abundant wood supply, don't buy wedges. If you can't split it, throw it back in thew woods and let it feed the bugs and worms. If you need every piece of wood you get, then good luck......wedges and a sledgehammer will work you to death.
My $0.02