There's a lot of people using clients from 2011 and higher, and they seem to be working good, from what I've heard.
Personally, I use 2010-07-21a for my server, so I wouldn't know how well the 2011 Renewal clients work, or how stable they are, but I'd guess (and please don't hold me to this) that an early 2011 client would work, since they'd support the extended character mounts, like Emistry said, and new effects, like you said.
And I'm curious... what do you mean by DLLs for those DIFF options? Like, so you can interchange DIFF options for a particular client on the fly?