I don't think getting consistency with SVG's filters is going to be a problem. None of the implementations for em and ex are consistent. Percent is broken in a couple too, including Inkscape.
@federicomena Let me know if you want the SVG test file.
A bunch of squares on a green background, different again again - librsvg
A bunch of squares on a white background, different again - Inkscape
A bunch of squares on a green background, different than the last one - Chromium
A bunch of squares on a green background - Firefox