We've been able to adjust our AAC encoders for CBR (constant bitrate) encoding. Almost no padding is added to compensate for the peak bitrate. See this topic for a previous discussion on the subject.

The PS and SBR parameters are automatically adjusted from the target mode and bitrate described below.

(HE-AAC v1) SBR is used for mono target from 16 to 48 kbps.
(AAC) None is used for mono target 56 kbps and over.
(HE-AAC v2) SBR and PS is used for stereo target from 24 to 40 kbps.
(HE-AAC v1) SBR is used for stereo target from 48 to 56 kbps.
(AAC) None is used for stereo target 64 kbps and over.

Please note that the AAC encoder (without PS and SBR) still use ABR (average bitrate) and padding is still use for this case.