I wonder how effectively they'll be able to use those funds since they have such a large target on their heads and every transaction on the chain is public information. Can they use enough coin mixers to obfuscate their identity?

Up to 40 BTC ($70k) now.

Considering it's only 40BTC, they could easily run it through multiple mixers in small batches and see good results on obfuscation. Nothing like the 5,500 BTC Tomas Jirikovsky tried to tumble and cash out.

However, I suspect WannaCry has negatively effected enough SysAdmins, that the interest in tracing transactions related to these addresses through the blockchain may be much more fruitful than traditional attempts. I wouldn't be surprised if the perpetrators don't even try to cash out, considering the risk vs reward doesn't make financial sense.