How does this MozJPEG 2.0 encoding compare to JPEGMini? I've been using JPEGMini for all client projects, it results in optimized JPEGs that work on all platforms with minute differences, rarely visible to the naked eye, only when I subtract pre/post in Photoshop. Very rarely does it save less than 5% and for larger images (saved with PhotoShop, save for web) often 15-30% or even more.
jpegmini works by compressing an image as much as possible while keeping the distortion calculated by there particular metric below a threshold. To compare against mozjpeg you'd need to compress a bunch of images with JPEGMini and then compress those images with mozjpeg so that they were the same size. You could then compare the quality of the resulting images.
mozjpeg does Trellis quantization, there is no detailed explanation about jpegmini, a guess is it optimizes the DCT similarly to how it is described in this paper:
Is the test set of images available?