Is there still going to be the bar/cabinet in the upper right? It just seems like that's a lot of wasted space with this layout both there and on the left by the stairs. Just to be able to think outside the box, does the screen have to be on that wall? It might be interesting to play with...
Put the 65" downstairs if you are happy with the 50" upstairs. You can move it up if you eventually upgrade to a PJ. What are you doing with the space under the stairs? That would seem to be a great place to put your equipment. Its right next to where you have the wiring and it won't take up...