Will merged mining mine each chain as efficiently as non-merged mining? Let's say I get a hash rate of X when mining either Bitcoin or Namecoin.

When using Merged Mining, will I be able to still do X concurrently on both blockchains, or will I do significantly less than X?

In other words, if I merge-mine other coins, will it hurt my Bitcoin mining rate at all?
Every hash you do will contribute fully to the hashing power of both block chains. You will have to do a tiny bit of extra work when you generate a work unit, but that's almost insignificant, and you only need to generate a work unit once for every few billion hashes you do. 

You will generate just as many Bitcoins as if you only mined Bitcoins and just as many Namecoins as if you only mined Namecoins. There are only two downsides of any significance. First, your Namecoin blocks will be a bit larger than they would otherwise be, since they include Bitcoin headers. Second, you will have to generate new work units every time a new block is discovered on either hash chain, so about twice as often. (This can be a big deal for a large mining pool.)

See my answer to this question for details on merged mining.