+1 for acoustically transparent screen. Then you could go with a larger screen and still place your speakers at an optimal separation. You'd also be able to get that center channel up to where it needs to be.
Also, how many people do you plan to have in the room. You won't hear this often, but if you truly only need seating for four, you may be better off adding some walls to make it a smaller room. While a large space is great to reduce the seat-to-seat variations in the sound levels, it also means you'll likely need higher sensitivity (pronounced more expensive) speakers. Of course, that's just speaking in generalities, so every case will be a bit different.
If you do reduce the size of the room, it might give you the opportunity to use the blocked off space for an IB sub.
If you could post up a layout of the entire basement, you might get some input regarding different layouts of your room.