I did some testing with a small acoustic song (wave -> mp3):
External Encoder: LAME 3.90.3
parameters %IN %OUT --alt-preset standard
results 143 kbit/s VBR file, min. bitrate 128 kbit/s, max. 320 kbit/s, Joint Stereo
parameters %IN %OUT --alt-preset extreme
results 156 kbit/s VBR file, min. bitrate 128 kbit/s, max. 320 kbit/s, Joint Stereo
parameters %IN %OUT --alt-preset insane
results 320 kbit/s CBR file, Joint Stereo
Internal Encoder (MC uses LAME 3.93)
VBR, Quality: High, Advanced: High Quality
results 152 kbit/s VBR file, min. bitrate 80 kbit/s, max. 320 kbit/s, Stereo
CBR, Quality: 320, Advanced: High Quality
results 320 kbit/s CBR file, Stereo
What surprises me, is that the internal encoder uses Stereo mode instead of Joint Stereo. It's well known fact that Joint Stereo mode with LAME gives the best results.
Anyway, it was quite impossible to hear any differences between those samples.
In listening test I used digital connection from the Terratec DMX 6fire soundcard (real 44.1 kHz, no resampling) to an upper class HT-receiver and the speakers were also quite high quality.