Charles, as you know I've already taken the plunge on completing my upgrade from v2 to v4. I am so glad I did!

In my room, I don't miss my two rear QS8s. I replaced my two side QS8s with QS10s and now enjoy vastly improved immersion, clarity, and special effects.

As for the LFR180, I am sure you know you'll need two amp channels. I think you'll also need it to be at least 6" away from the wall and not in a cabinet.

Is it the rear QS8s you want to replace with M5s?

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated