The mainly problem here it isn't the less bitrate of the FhG enconded song vs Xing.
The problem is that Xing codec has a really bad implemented psymodel that only cares about speed, and it has many flaw in its design. This means problems with the encoded music independently of the bitrate

.
For example Xing doesn't use short blocks with the problems associated with the no use of this technique, doesn't matter the bitrate you use.
Do you have to remember that FhG and Lame are good implementations of a mp3 encoder and really fine tuned, (Lame with alt-presets).
Hence, a FhG or Lame could result in higher quality than the same Xing mp3 no matter what bitrate due the inherent faults in the Xing encoder.
Forget XingĄ