I think he's referring to the wall to the immediate side of the speaker, not the distance behind the speaker. In answer to this, I don't know how it would affect it.

Cat Brat; just specifically for music... I agree not optimal for HT. I have a dedicated HT in the basement that I use for games/ movies and such... at the moment all run with Polk audio...(opportunity for future changes!)

If money was no object I would eliminate the nook and make a customised shelving for AV equipment, pics/books etc.; change the mantel to allow for lowering the TV, add the addition of a centre channel. This would enable me to build cabinets for and procure in-cabinet M80s for either side (maybe a foot to either side of the fire place.)

Not sure if I have the relationship capital to undertake that project (not to mention real capital!) so an interim solution is required. Hence the idea of the M22s with sub...At first I thought of the on wall/ in wall but now I am not sure. they would look odd stuck to either side of the mirror. Now I am wondering if the M22 bookshelves with the current stands would be acceptable (run the speaker wire in wall to maximize WAF.

Actually Dakkon was right I was referring to the amount of distance from the wall to the back of the speaker. Essentially I am looking at the same setup in the earlier posted pic but replacing the ProAcs with the M22 bookshelves now... again not to be used for the TV and I will not be placing any speaker to the right of the TV...

Sorry for the confusion and thanks for all the suggestions. Some of the confusion is due to me changing options on the fly and me not originally providing a picture!
I really appreciate the input and patience!