CSMA/CD only requires that you back off if there actually is a collision. CSMA/CA additionally requires that for every frame sent, after sensing the medium as clear, that you wait for a random amount of time before sending it to avoid collisions. If the medium is frequently clear, CA will still have the overhead of this initial wait where CD will not.
Depending upon how it's actually implemented, CSMA/CA may have the same (untended?) behavior of CSMA/CD in the sense that setting TCP_NODELAY will also set the backoff timer to zero. It would be interesting to test.
