The way I see it is that the material is only half the story.

Going back to Bitza 2 vs Little hitter, I know that the metal on the top is only supported at the edges. What I suspect is happening is that as the material isn't held stiff and can bend it is wasting some of little hitters energy and as a result, not getting punctured.

Conker 2 is designed in a similar way. The 2mm Ti armour could probably get punctured by little hitter or NST but it is mounted on a layer of HDPE and has rubber washers between itself and the chassis, giving it some room to flex.

As for the Aluminium, Solidworks 2013 give the following properties for various T6 Alu's...

4032-T6 : Yield Strength: 315MPa, Tensile Strength: 380MPa, Shear Modulus: 26GPa, Mass: 2680Kg/m^2
6061-T6 : Yield Strength: 275MPa, Tensile Strength: 310MPa, Shear Modulus: 26GPa, Mass: 2600Kg/m^2
6063-T6 : Yield Strength: 215MPa, Tensile Strength: 240MPa, Shear Modulus: 25.8GPa, Mass: 2700Kg/m^2
7075-T6 : Yield Strength: 505MPa, Tensile Strength: 570MPa, Shear Modulus: 26GPa, Mass: 2810Kg/m^2

As you can see the Alloy has a massive effect on the strength. 7075 Is typically for high strength engineering where as 4032 is used for high tolerance applications but is pretty useless structurally. .

If you want to stop spinners you could welding up a cheap steel shell and then having it Nitrided or case hardened. Each has its own benefits. I am sure someone on the forum could do it for you on the cheap; we have our fingers in many pies after all!