A butt-load spent in cables - how much improvemt?

I'm very happy with my current cabling

Me too...I use stock PC's, stock balanced XLR IC's and Monster speaker cable for surround speakers. I do use a power conditioner to get rid of AC line noise and to maintain appropriate voltage and it did make a small difference but apart from that I don't sweat or worry about what kind of copper wires to use between gear or those that connect them to an outlet.

If simpy changing a cable makes a huge difference then it says more about the robustness and matching of the system components themselves than anything else. Good properly matched gear should not change presentation dramatically because of a different piece of appropriately shielded/sized wire that connects it to other components or supplies power to it. Just my two cents.

Great setup. Those alcoves for the speakers/sub might pose some significant bass management issues have you considered adding bass traps in the corners at the back of the alcoves?

I see the fronts of your speakers are brought forward from the edge of the wall- so edge diffraction should not be so much of an issue...nevertheless you might get a better imaging withe the speakers forward a bit more.

Strictly speaking you should close the gap between the frame and the center channel (make sure no gap) - a picture frame sometimes works well - a gap will give you edge diffraction issues and reduce the imaging.

I really like this room and setup...it looks awesome and that 71" screen has me drooling!! I hope my suggestions are taken in the spirit they are offered...just passing on some experiences...suggestions only and no criticism intended on what is one of the more impressive HT setups on A'gon. nice job.