I don't quite get this part. You'll calibrate for the plasma too, I presume.
If I had to guess, improvements will be easily discernable. I've hung out in the PJ and screen forums, and have picked a brain or two as well. If there was any boundary that was most important to treat for light, my guess is that the ceiling would be voted #1. Also, there are knowledgable folks who believe that treating the back wall is a great thing, which I guess is already the case with you, but driving home a point (because the ceiling will have more effect than the back wall).
New screen looks great!