The vast majority of Inkscape filters appear to be direct implementations of
SVG filter effects written in XML. Their functionality already exists, for the most part, within GIMP; it is just that the names are different and the parameters arranged differently. (GIMP predates the standard so it does not comply with it exactly; sometimes for better, at others for worse).
Note that once GIMP is completely converted to
GEGL (planned in 2.10), support for SVG filter effects will be built in and replicating Inkscape filters exactly should become trivial.
If there are specific filters you'd like to see converted, please point them out. It appears that most of them are already available directly through GIMP operations.