I actually don't know why the default shader is called just that, but I'm used to the terminology now. It's used in all objects, and it's far more work to import image maps for each, so I would welcome an extra button. So, diffuse too, mmm, never tried that.
Bit depth is 8, which would give 256 gradations. I think that would be enough for a few millimeters/centimeters of bump. Are you using more bits?
Working from Lightwave I have to unmerge all points or my texture maps go awry in Poseray, stretched strangely sometimes. Then in PR I change all groups to materials, merge points and recalculate normals for each material (some of course need higher angles, like draped cloth). Standard is 35º. Maybe recalculating isn't even necessary, but the texture (as said) comes on differently then.
But with walls you also have inset windows, where I mostly use only one 45º angled polygon to soften that angle a bit. So then I need to increase the normal angle, and sometimes get into trouble if some areas in one material should be 'flat', others have a corner to soften.
But now I knifed the areas just above the windows once more and got rid of the nasty 'shadows' in those elongated triangular polys. Though I probably should have built it differently from the start. I still have to learn a lot, I guess.