I had a huge improvement in image quality by going from the stewart 130 to the stewart 100. Given your large size screen, I would consider the Da-lite JKP Affinity with a 1.1 gain. When the time comes, you can get samples of all three and make your detailed inspection of the sharpness and brightness available with those screens at 4K.
You could also go with a Stewart Cime Neve 1.1 screen. I have one coming for my theater in a week or two.
Edit - I wouldn't go with a 1.1 screen with your current size though - too big, not bright enough.
Edited by Craig Peer - 9/29/13 at 5:11pm