I got my brake hanger back today after my friendly fabricator Indiana had shaved off some 6 mm, leaving the full 15 mm only where the brake caliper bolts to the hanger. These are my findings on the subject of laser/abrasive water jet-cutting of thicker alu pieces:
"Daws" mentioned that the program I'm using – Adobe Illustrator – is not very good at generating AutoCAD-files and that the resulting curves are not as easy for the cutter head to follow as curves resulting from a proper CAD-application. And he does have a point. Looking at the cut surface (see image 1) it's obvious that the cutting head/water jet hasn't described perfectly smooth radiuses (radii?). It looks a littble bit as if the cutter head has paused/started/wobbled around a bit. This may of course be a characteristic of the water cutting method, but I suspect that it has something to do with the file as well. He also cautioned me that the cut surfaces won't be 100 "parallel" but rather have bit of taper - also a characteristic of the cutting method. Again, he was correct. It wasn't super obvious though and I'm not sure I would have noticed if I wasn't looking for it.
So, is it a worthwhile method? Yes and no. It's a quick and relatively cheap way of turning out pretty complex shapes without having to resort to more complex CAD/numerically controlled milling. On the other hand, the resulting pieces are nowhere near as perfect as pieces done using a mill. I spent three hours cleaning up and smoothing/straightening the edges in order to make it look like I wanted (see image 2). The surface isn't perfect yet – I'll leave the final polishing to the anodizers.
Conclusion: I think I'll save this method for thinner pieces in the future. And I'll use the freeware application "Solid Edge 2D Drafting" that Daws recommended.