It really depends on what you mean by dead space. If by dead space, you mean space under the false bottom, then it is not a big deal. If by dead space you mean volume that is not drained from the mash tun, then 11 quarts is a big deal. Dropping your false bottom won't help with that though, which is why I think you mean the first version. If it is a problem of not draining, adjust the pickup tube so that is reduced. If it is extra water under the false bottom, drop the false bottom if it is easy, if not no big deal. It's a lot, but it's not the worst thing in the world.