cwebp has a commandline flag of -alpha_q <x>% which uses lossy-compression for the alpha channel and can significantly reduce image size, and also seems to speed up image conversion in some cases. It would be great to have this ability in Pillow also.

Additionally an option similar to -noalpha where we know the image does not contain any alpha layer saves ~27 bytes per image - not particularly significant but again a nice-to-have for cwebp compatibility.

Thanks!