Most of the time Sound check is not worth it as it eats your iPod battery, the battery life will reduce. I think things like volume adjustment and sound check are for people with sensative ears that can't really stand loud volumes. The volume increase is also helpful for people that are hard of hearing. Either way, the average user does not need them.
melsmusic, whoops is right. For example, a 192 mp3 is about 75% quality of the original CD. This is a high estimate but work with me. So say you convert that file to a 128AAC file, which is 65% of the original quality. By the time you are done converting, you have a song that is about 35% the original quality. The same thing if you are converting from 192 mp3 to 192AAC, you will have a song that is 37% of the original quality.
64GB iPhone 5 | 64GB iPad mini | AppleTV 2 (2012) | AppleTV 2 (2010) | 2012 15" MacBook Pro, 1TB SSHD, 16GB DDR3 1600 MHz, OS X 10.8.3 Mountain Lion | Apple Lossless | iTunes AAC 192kbps VBR | iTunes 11.0.2| Library size = 1.04TB | Legacy iPods: 3G 40GB, 4G 40GB, 5G 60GB, 160GB iPod classic (2009)