I just submitted a patch (id: 1896251) to the patch tracker that changes Inkscape::SVG::PathString to generate the shortest possible path string with decent spacing (that is, it uses the same spacing rules as the old PathString). It does this by eliminating repeated operators when it can, as well as using relative coordinates if it helps.
Using this patch I got file size reductions of about 10% on car.svg(z), the new about screen and some of my own files (I looked at both the uncompressed and the compressed svg's). To do this I opened the files, selected all objects and performed a translation of 0px on them, then saved the file (as uncompressed svg, and then again as compressed svg). The optimized path strings are about just as readable as the old ones (sometimes they're even easier to read).
I would welcome any suggestions/comments.