The only thing I see missing from your list is,
room treatments. I believe your question as been answered

. On the audio side, it will make a much more discernable difference in sound quality compared to upgrading the AVR. Regarding adding a sub, if the current one is sufficient for the main listening position, the primary gain from adding a second one will be a more even low end frequency response in more listening positions (and perhaps some improvement in the main position based on placement and settings).
On the video side, you can use
this calculator and determine, based on screen size and viewing distance, if you will benefit from upgrading to 1080p.
Incidently, care to post a pic of your M3 in the Steam Vent, car thread

?