Not sure it would say you made a mistake, I owned M60's for many years before getting the M80's, they are both awesome speakers and very similar.
1) I don't think you will see a HUGE difference, they have made some changes to the crossover, tweeters, and a few other minor changes through the years. The m80's will give you a little more in the bass regions, and possibly be a little more detailed.
2) Don't know I have the vinyl.
3) Again, following the answer to 1, the changes have not been HUGE. I think you might see some subtle differences but not Earth changing.
4) The only thing I would change is that your Qs8's should be "side" surrounds, not "rears" as you mention. Hopefully, you have the placed correctly. Other than that things look fine.
Do you think your system is lacking in some way?
