I've got a good trick for producing some nice unclean, natural looking dirt maps.
I simply download some pictures of dirty brick or concrete walls, stones, etc (Myang's free textures is an excellent resource for this). I take the photos into Photoshop and crop them, then convert them to black and white images. I've found that when applied to, say, a brick wall texture in Maya, they give an excellent natural dirty look to the wall, making it look instantly more realistic.
To fix the deforming textures on the roof you need to try planar mapping the surfaces. This is under Edit Ploygons --> Texture --> Planar mapping. Just adjust the manipulator to your liking, or until it looks correct. You could also UV map the roof/wall, then put the UV map into Photoshop and paint the texture using the map for a guide. You can get more info about UV mapping and texturing from the Maya help files or www.learning-maya.com.
I've tried using Final Gather, and it can produce amazing results. Just by playing around with the values you can get pretty varied results. To reduce rendering time when it comes to the paint effects tree which is converted to Polygons, try reducing the tree's poly count, as it is probably sky high! Alternatively, render a paint effects tree out normally, then map it onto a plane in the foreground, or comp it in afterwards. This way you don't need to have the actual geometry in the scene.
Hope some of this helps. Your doing a good job tho, keep going!