You could always make a new and deeper mantle.
Or go without a center channel.
This is one of those home design things that I have a problem with. TVs over the fireplace.
The problem is that many houses, including the one I'm in now, have the fireplace as a massive "feature" in the living room. I personally hate fireplaces, think they're just a massive hole in your house's envelope. But that's the way it is. And really, I'd love to be able to put my TV above the fireplace, as then the furniture doesn't have to be "crooked" in the room. But to do this, I'd need to drop the massive surround of the fireplace down and maybe decrease the width, and that'll take too much money.
By the way, I think this is like fancy tubs in the master bathroom: does anyone really use these? We have a massive one with jets in our bathroom. I've never been in it. We put the kids in once, put the jets on, and they freaked. And the amount of time you have them in tubs is really not that long.