Interesting problem. My first thoughts were 5.1.2 max for that layout, and I too wanted to see you turn it to be long and not wide... In V2.0 of my theater, it was super wide and only 12 feet from rear wall to screen. My wife loved that idea as it was in a shared space, but I hated all of the challenges with not having usable side walls, and not being deeper away from the screen.
I then switched to V3.0 in the same house. Different room, and it had its own problem, but I was able to get 15 feet from rear wall to screen, and I actually had side walls now. The sound was light years better, as was the overall experience, using the same speakers, receiver, projector, and screen.
Now, you noted that your family could tell a difference when adding more speakers, but will you get a huge benefit using your current layout? Maybe.
Sure, more speakers are better for imaging (as long as the speakers as assignable, and the room is "good"), but there is so little space behind your seat that I am not sure that it is really worth the effort.
A few things that could help your overall sound if you absolutely can't rotate 90 degrees in the room is, as already mentioned, bump your L/R speakers away from the wall a little bit. Let them breathe!
Take your side surrounds and actually position them slightly in front of your listening position. This will help to fill the "gap" between your left/right speakers and the next closest "bed channel" speaker, you side surrounds. It will also put some separation between those and the rear speakers should you go that route (with the current in-walls, or using Axiom in/on-wall speakers).
You really don't have great placement options for overhead speakers. You can put 2 speakers up on the ceiling, just in front of the soffit that you have there and get your biggest bang for your buck in your setup. If you wanted to go one step further, you could put 2 speakers up front as high up on the front wall as possible (in more of a DTS:X position), but again, you won't get as big of a benefit.
Again, with the limited space from the rear wall and screen, I don't see a big benefit over 5.1.2 or 7.1.2 if you can get the rear speakers ironed out.
Don't give up though, just more to think about. I could talk more about not using QS speakers in an Atmos layout, but it is fairly common now that monopole speakers are the most ideal, especially for smaller spaces like yours. In large rooms, QS speakers *could* be OK, but that is all I will say about it.
Keep talking through it and see where you end up. I am loving my 7.2.4 setup, but I have a purpose built room (14'wide x 24.5' deep, soundproofed, lots of acoustical treatments, and all monopole speakers.