as it stands right now, Sunder is pretty much useless, not only do you have to be able to connect the sunder attempt but they also get a fort. save against measly -4 to AC for like 30 seconds or so
why not hype it up a bit and make it so that a sundered monster will lose 50% fortification while sundered?
while we're at it, make it so the shatter weapons have a chance like Weighted weapons to auto sunder the enemy
as for Improved Sunder, make it so that it gives -100% fortification for the duration