Did someone conduct a test on how much the quality will degrade when transcoding from 320cbr?

I have a couple of 320cbr mp3s encoded with lame3.87. I can't put my hands on the original cds and 320 is a way too high for me. Now, the question is how much the quality will degrade if I'll transcode to --aps standard. I know that it'll, because of introducing additional noise as a result of double quantization, but a question is if it'll be realy audiable and in what form?
Are there any better switches for transcoding than --aps standard?

10x