[final kaoss]

Yes by default google does enable lossy compression. And in general it usually means a loss in quality when reducing the image. But lossless compression can reduce the file size without sacrificing any noticable image quality.

Also some mods aren't friendly when it comes to combining, deffering or compressing JS. I recommend playing around with the settings until you find out which works just right for because everytime you change the settings the pagespeed bot will recrawl your site, looking again for js resources and making the applied changes.