I'm guessing there should be no objections to use of .cljc files in a contrib library causing minimum supported version of Clojure to move from 1.5.1 to 1.7.0?

no

but you need to let me know when you do that so I can adjust the build box test matrix

Will do. No permissions to do anything yet -- just making plans.

Are CHAMPs better (faster) than HAMTs in every way (as some proclaim)? If yes, is it possible/how hard would it be to replace the current HAMT impl with those. Although it's not a part of the public API of Clojure, people might rely on details of current impl and be broken. What kind of other considerations exist here? Apologies if this is noise.

Don’t know

It would be a large effort to answer those questions and be a convincing effort to change

When people have looked at it they found champ was using a different hash function which had an impact on comparison