playing around, testing with new ffmpeg (from git), only interested in vbr modes, this looks like it might work:
for lc:
ffmpeg -i in -c:a libfdk_aac -flags +qscale -global_quality 2 -afterburner 1 -vn out.m4a
(valid range for -global_quality seems to be >1 (1=error))
for he:
ffmpeg -i in -c:a libfdk_aac -profile:a aac_he -flags +qscale -global_quality 1 -afterburner 1 -vn out.m4a
(valid range for -global_quality seems to be 1-5)
for he_v2:
ffmpeg -i in -c:a libfdk_aac -profile:a aac_he_v2 -flags +qscale -global_quality 1 -afterburner 1 -vn out.m4a
(valid range for -global_quality seems to be 1-2)
-------------------
Real world speed test on entry level mac-pro (Lion), encoding one album from flac source:
case1: flac > fdk_aac (-flags +qscale -global_quality 2 -afterburner 1), time = 02:53 (slow), without afterburner = 02:52 (slow)
case2: flac > intermediate aif > apple aac (afconvert -v -f "m4af" -s 3), time = 01:10 (fast)
(note: this speed test is small and most likely irrelevant, could also be some mistake I did during compilation of ffmpeg with fdk)
-------------------
about ffmpeg used:
ffmpeg version N-43145-g32aeba1 Copyright © 2000-2012 the FFmpeg developers
built on Aug 2 2012 19:14:22 with Apple clang version 3.0 (tags/Apple/clang-211.12) (based on LLVM 3.0svn)
configuration: --prefix=/Volumes/tempdisk/sw --enable-gpl --enable-nonfree
--enable-libx264 --enable-libfdk-aac --cc=clang --enable-runtime-cpudetect
libavutil 51. 66.100 / 51. 66.100
libavcodec 54. 47.100 / 54. 47.100
libavformat 54. 22.100 / 54. 22.100
libavdevice 54. 2.100 / 54. 2.100
libavfilter 3. 5.101 / 3. 5.101
libswscale 2. 1.100 / 2. 1.100
libswresample 0. 15.100 / 0. 15.100
libpostproc 52. 0.100 / 52. 0.100