The new decoding library has a new fast seeking system. It was discussed here:
http://www.hydrogenaudio.org/forums/index.php?showtopic=50130. Sluggish seeking has been the biggest problem with Musepack.
In that thread I asked if updating the source libraries would be difficult:
Would building a new decoder plugin for a player program be exactly similar process as before? I mean, could a developer just take the new library and repeat the same steps he/she did with the previous decoding library?
and got this answer:
They seem to use libmpcdec-1.2.2.
Libmpcdec-1.2.3 is backwards compatible with all 1.2 versions, so unless they modified libmpcdec sources, updating their plugin should just be a new compile with fresh sources. No code-source modifications needed.
IMHO, you should update it. The MP3, Ogg Vorbis and WMA formats are now up-to-date. Only Musepack is not.
If you decide to update remember to check how the discussed "fast seek" switch can be enabled. I have tested it with the current foobar version. It works perfectly fine and seeks much faster than the old decoder.
Naturally you should update the encoder too. Just replace the v. 1.15 exe file with the v. 1.16 exe file. (Currently I do that manually after installing each new MC12 build.)