Here it is: If you have the original uncompressed cd's of the music, you can do Apple Lossless and it will benefit you from having the original 1411kbps bitrate of a CD.
In your case, anytime you have mp3's, aac's, that's the end of the story. Not saying mp3's or compressed music sound bad, but you can't make music sound any better converting to lossless when your file's have already been stripped of the information.