Well, the C100 is pretty similar at first glance. But it does have a wave guide for the tweeter which sets the tweeter back a little, which is probably an improvement. But other than these differences, it seems like only a small step forward. But you never know for certain unless you try it.
The only center speaker in the ML lineup that will be a huge leap up from any of the models other than the C34A is the Focus C18. The gains are there to be had when comparing the other center speakers to one another, but the difference in sound will be incremental. The painful part of where we are today is that the prices are way higher now than last year!
I agree with you that the C100 may not be worth the money for the upgrade in sound vs what you now have.