With the current trend of actually selling MP3 files. I guess it's good to understand as much of this as possible.
Sadly, I have not gone into it that deeply though I've been getting very good results within the medium's limitations...
I export my mixes as 24/48 and open them into a new Sonar project setup for mastering..
When I'm happy with my mastering I export as 16/44.1 wav as well as (using Sonar's MP3 dialog) a 320 stereo, high quality MP3....
I don't expect them to sound the same, but as long as it seems to resemble the mastered wav file effectively, I'm satisfied. It seems to me that the majority of MP3 files sound less quality due to heavier (lower) bit rates...
I would rather people buy the wav files. I played with modified mixes specifically for MP3 conversion and could force more elegant files, but then they stick out awkwardly in most MP3 collections. So I stopped.
Dropping the gain does give a touch more clarity, but again the offset here of volume wars makes it a moot point knowing that especially with MP3 collections, the listener is not putting on your album and setting playback. The listener is setting playback and hearing playlists.... All need to conform somewhat.
So I've been making my mp3's at -0.3 and continually get praise for my MP3 files sounding good...
If the MP3 sounded as good, they'd gave less reason to buy the album for higher quality, eh? ;-)
They're a hardly medium for passing around work versions of songs and the likes, but though I alliw sale of them I would much prefer to have listeners buying higher quality...