I ran diagnostics on my iPhone and saw:
A2dpStreamConfig.cpp:598 PickStreamConfig Info A2DP Selecting remote AAC SEID 3
A2DPClient.cpp:1118 SelectStreamConfig Info A2DP Setting stream configuration for local SEID 4 and remote SEID 3 on device xx:xx:xx:xx:xx:xx "Tesla Model S", remote delay reporting 0, local delay reporting 1
...
A2DPClient.cpp:1260 SetConfigCfm Info A2DP Attempting to open streaming channel to device xx:xx:xx:xx:xx:xx "Tesla Model S"
...
A2DPClient.cpp:1330 OpenCfm Info A2DP Successfully opened streaming channel to device xx:xx:xx:xx:xx:xx "Tesla Model S"
A2DPClient.cpp:563 MediaConfig Info A2DP MTU for device xx:xx:xx:xx:xx:xx "Tesla Model S" has been changed to 1005
A2dpStreamConfig.cpp:708 SetAacDataRate Info A2DP AAC frame len: 743 bytes, num frames: 1, bitrate: 256 kb/s, rtp intr: 23219, rtpFrameLen 990, latency 92879
A2DPClient.cpp:2630 PublishAudioDevice Info A2DP Creating A2DPAudioDeviceModel for device Tesla Model S with handle 1
A2dpStreamConfig.cpp:102 GetCategory Info A2DP Publishing device as carkit
So, it looks like 256 kbps AAC was successfully negotiated over Bluetooth!