No, not in a meaningful sense. Validation is very cheap. You do run into block size limits if you're trying to transact too much at once, but any privacy system is limited in its privacy by transaction volume.

"Dark Coin" really strikes me as pointless. The whole idea in coinjoin is that coinjoin is already part of the design of Bitcoin. There is no advantage in having a new and different system. If you're going to do something incompatible, losing Bitcoin's network effect in the process, then you can do something much stronger.

It also depresses me somewhat to see people talking about darkcoin (or even zerocoin/zerocash) when bytecoin has a privacy system with much better properties than CoinJoin (it's similar to CJ except you safely join with offline coin holders, and all users are participants), something made possible by the fact that it doesn't have to fit within the existing Bitcoin network, and it's completely practical, reasonably performant and deployed for some time now. But strangely, it's virtually unheard of...  Bytecoin's privacy properties are in some sense weaker than zerocoin's— since its like a supercharged coinjoin— but the cryptography is much stronger and much more efficient, so in practice I'd expect it to have better anonymity just due to it being much more practical (also as evidence to it existing as a deployed system).  ... so yea, if you actually are interested in privacy technology in a non-bitcoin system, Bytecoin seems to have pretty much nailed it.