Yep, at that point the only thing that's changing is the format and filesize. The actual audio data cannot get better at any point in the equation. The WAV will sound exactly the same as the MP3 -- or potentially worse depending on the converter and the process used!
For the 16bit vs. 24bit .. it's possible to go from MP3 to 24bit WAV, but due to the reasons above there's no real reason to do so, it just makes the file bigger once again. If someone needs to bring it back into a 24bit project give them the 16bit wav and let them do it when they import it.
Now if you want a compressed format that is lossless (i.e. you don't lose any sound quality) go with Monkey Audio (.ape). It works in a much different way than MP3 and doesn't destroy any sound data during the conversion process in or out.
Hope this helps
Mark
P.S. Good to see you post Johnny!
It's been a while hehe.