Don't give Jake such a hard time Wouter, the problem with your spreadsheet is that you could calculate that a chocolate boom can be made as good as a carbon one, Yes it would have a large diameter and small wall, just like your aluminium one would.

When you go on to say "a wallthickness of 0.1 mm is impossible to achieve" I think that also makes your post pretty meaningless. (although I think you meant 1.0 mm)

In general terms most engineering materials except carbon have the same stiffness/weight ratio.

Carbon is the worst material for lazy engineers, because it is not homogeneous and can have different properties in different directions. It is a very hard material to design with. But can be amazing when used correctly.

As far as the efficiency of materials goes, you make a good point about efficiency of materials that are volume constrained, but most of the engineering on a cat is weight constrained. The only volume constrained case on a cat I can think of is where you are deigning for least drag, and then it is only really applicable with situations where the material is purely in tension. Like Dolphin striker support and stays.

Gareth