Hey Peter,

My post was actually two separate statements.

The mirror was one.
The example I've done a 0.8 was the other.

Something else you can try would be a different projection screen medium. Part of the reason the image looks perceptibly darker to the edges is that your eyes are not directly on axis with the lens of the projector. Some screens account for this and others do not. Typically the ones that do have characteristics similar to road signs where no matter where you're shooting from, the image always seems to reflect back toward you.

In your case, try using a different screen medium to see if the brightness falloff goes down. Also, if you have visual distortion, use the spherical correction tools in the Mix windows.