Exact same build with a normal great hammer, it's great.
Swings are often blocked, but I missed only one crushtrough out of hundreds with overheads.
Really usefull and efficient, but try to have a normal sword (Sword of war... wathever you are confortable with) to deal with 2 handers, ppl who could outspam you, or for duelling in general. The other solution is to become really good with chamber, ppl become way more predictable when they see they can just spam the shit out of you.
I'd be interested to know if you often crushtrough when swinging though, I don't know if heirlooming the GM is worth it.