Variable Bitrate in AAC

Hi.
I’m using LameXP with NeroAAC codec to get the best AAC files from FLAC sources. NeroAAC allows to encode in VBR with bit rates above 320Kbps. Maybe ABR at 320Kbps is enough for stereo files, but with 5.1 tracks the sound quality is degradated.
Can you add an option to select de bitrate model.
Thanks!!