Great script Graechan! I made a few tweaks to my layers. Don't know if you want to incorporate or not but here's what I did.
1. Isolated the grout on a transparent layer and gave it a 7E7E7E gray color, duplicated it and blurred the dup at 5, bumped the original at 12 (inverted). Then deleted the blurred layer.
2. Made a new white layer inbetween the grout layer and the image layer. Filled it with RGB noise @ .50 and set the layer to multiply then duped the layer to give it a gritty texture.
3. Duped the image layer and blurred the dup at 15, bumped the original image layerat 7. (deleted blurred layer)
4. Did an alpha to selection to the image layer and applied Glossy Stroke 3D set to Fill at 1 and turned the opacity of the shadow to 0. deleted all the glossy stroke layers except for the one named "glossy layer", then did a simple Blur>Blur on the glossy layer.
I got a very shiny look almost like glass tiles and a little better recessed look to the grout. I got a little bit more than I wanted in the upper right hand corner of the pic but I still liked the results. You can see the grout texture layer really well in the lower right corner.
Now that looks fantastic Drac; might try the glossy-stroke idea later (usually use Inkscape; why not do it all in GIMP. lol). Thanks for the update too Graechen.