Recently I upgraded to 1.0.1. In the process, all my images were over-written. Many of the XF images are pretty well compressed and the Smush.it process only offers a 10% savings. Other images offered an 80%+ size savings which is huge. For example: styles/default/xenforo/color-picker directory has multiple images which offered 80%+ compression savings.
If XF compresses all their images, it will be a win for the product. It shouldn't take much time overall, and on a personal note, it would make the upgrade process easier