Comment made by: hiredman
It seems like the best thing here would be to change sort to return a vector. Usages of sort in the middle of sequence pipelines will continue to work, but a sort followed by conj will break (I cannot recall an instance of this off hand, but I am sure they exist). Sorting seems to imply a fully realized collection, and vectors are the "strongest" realized collections that can be returned here.
Given the conservative nature of core, and the issue with conj ordering above, the next best thing might be to add a sortv similar to the existing mapv.
Another option might be to remove the call to seq, so sort returns the sorted array. This would actually be really useful because you can use Arrays.binarySearch. Calls to conj after a sort would then fail with an exception instead of conj to the "wrong" place.