I'm thinking you would be better off working on the left/right walls versus the ceiling for addressing the "first reflection points".
For me, I would not worry about a panel behind the center speaker, but that is just me. If your back wall is more than 10ft from the front wall, your never going to get reflections bouncing back that this panel would address anyway, or that your ears would notice a difference.
Also keep in mind it is very easy to build your own panels if your handy and save a lot of money.
Randy
