Last Tuesday morning (11 March) I arrived to T1 on BA 56 at about 6am from JNB and connected to AA 57 departing from T3.
Even though I had a FastTrack card from BA, I used the IRIS station at T1 (never experienced a line for IRIS), left the terminal and had a nice 7 minute walk to T3. When I arrived at the AA counter at T3, they confirmed my bags had been transferred.
I always avoid using the FCC.