ANW said:
...how much abuse and water restiance can type 2 HA take?...
In this context, I think there's not much
functional difference between the two. Bare aluminum is waterproof and pretty corrosion proof except maybe for hot salt spray. Abuse which affects function of the light, e.g. deforming the metal, will pretty much be unaffected by any type of coating.
Type III HA provides a much stronger
surface coating to the aluminum. In many applications - HA aluminum cookware and bearing surfaces inside an M-16 to name 2 - this is critical so a high quality HAIII coating is applied there. For flashlights, the improvement is primarily cosmetic so Type III is worth about as much as you value the appearance of your light.
Also, HAIII coatings vary in quality and thickness. I don't know if even Surefire can match the HAIII you'll find on a Calphalon HAIII skillet or a US GI M-16.