AAC Encoder in iTunes 7.5: significantly improved!

Question

Upon the reported 'problems' with the AAC-encoder in iTunes 7.5, I did a short analysis upon a self-created reference track (amongst others, containing lots of silence) @ 320 kbit, (supposedly) CBR AAC and MP3 encoding, and compared these to the previous version of iTunes. Here's what I found:
AAC:
1. Silences are encoded much more efficiently, leading to a reduction in file size (in this case) of about 50%
2. The residue (the AAC-Waveform 'subtracted' from the original waveform) is four times smaller compared to the iTunes 7.4 encoder. (Although this is no real measure of the encoder's quality, regarding psychoacoustics and all that!).
3. Furthermore, the amplitudes of pre-echoes, a significant compression artifact, are also reduced to 50% compared to the previous encoder.
As for the indicated bit-rate, it's just the average for the entire file. Regarding the above, even if it indicates less then 320, you still get more.
MP3:
1. The better encoding of silence has also been applied in the MP3-encoder, however only leads to a reduction of file size of about 25% (this is probably the maximum the less efficient MP3-format allows).
As for the rest, the MP3-waveform from the 7.5 encoder (including all its artifacts) has remain ex-act-ly the same compared to 7.4, really, to the last bit. So the benefit only concerns storage here...

Answer 1

The codec is closed source, so there's not a change log (that I know of) to look at. Probably the best source of information on the status of iTunes AAC would be in the way of listening tests on Hydrogenaudio.