Ok, it DOES look like we want to talk about the same setup, facing eastward, the viewers I mean facing eastward. I don't see any issue (save for one, will address in a second) with being in a wide area when you are that far back; I believe the left/right balance WAS a greater issue, but now that I think about it, if facing westward, and you were given quite a bit of freedom in pulling the speaker out from the corner, then I might go that route. The boundary imbalance will be mitigated, and 10' with the other way is kind of getting a little tight for three speakers. Perhaps the boundary balancing is simply balancing too much boundary interaction from both sides to begin with.
If you still go for the eastward route, but want to sit THAT FAR BACK, the issue IMO is simply the SPL levels. You would need some big and very ugly commercial/pro type speakers for anything resembling movie theater levels. Now if you just don't need that, then you don't need that, but 40' is a huge length. SPL drops exponentially with distance. I've seen some pros think that once at the around 25' or so, you need upwards of 20K to make it work, dunno how true that is, but distance is an issue that way.
Too high is definitely in a compromise, you will for sure want to angle them as much as you can. You know how I feel about the corner. The further away you can get, the more I'm okay with it. You know what you ought to do is just throw the speakers in both locations and give it a listen. That's the best way, only YOU will know exactly
how far you are willing to place any given speaker from any given boundaries, and then you can listen to that result. It will for sure be a lot more tamed once all the furniture and materials are brought in, but you'll still get a much, much clearer idea. I simply assume acoustic panels and bass traps are simply a no go here, but if I'm wrong about that, the corner becomes less and less of an issue. It might not be bad anyway, and some people are pickier than others, some people imagine things more than others . . . let us know how your experiments turn out.
Consider my 2 cents given!