The keystone correction doesn’t currently use the entire width or height of the projector’s DLP chip. You can see this in a dark room, there is a 5-10% border on the left and right that is unused, regardless of the keystone angle. And if you set the projector at an angle up against the wall, up to 50% or more of the vertical height of the DLP chip is wasted by black bands at the top and bottom of the image. Cropping at the sides and top of the image is not just a waste of DLP pixels and resolution, but it also decreases the total light output of a video frame.
I would really like a keystone correction option that allows for the image to always be stretched to the full height of the sensor, and clipped to the edges of the sensor if necessary. This way, the image content can still be keystone-corrected by trapezoidal stretching, but the maximum number of DLP pixels will be used. On the wall or screen, it may look like there is an “upside down keystone” cropping applied to the sides, which is the DLP chip’s projection footprint, but the content will look correct.