Overhead has not been measured yet, right? Also, not that with pypy this isn't so much of an issue since you don't have to thunk back to python from C.
I don't mind dropping simplejson. Wrt Python 2.6, we can be 2.6 *compatible* but saying that it won't give you the best experience (since stdlib json on 2.6 is pretty slow).
Overhead has not been measured yet, right? Also, not that with pypy this isn't so much of an issue since you don't have to thunk back to python from C.