The edges of an anamorphic image on film are not bowed/pincushioned; it is a square image. The content of the image itself may have lens distortion, but that is a separate issue. All my sources are anamorphic, and do not exhibit this behavior when unsqueezed. I've generated anamorphic visual effects for film outs hundreds of times as well, without bowing/pincushioning in the image. A compensatory curve of the screen in a theater should be necessary only when the projector's lens itself adds a bowing to the image, but the actual film frame images are square.
_Mike