hi tony-
aluminum is something i certainly considered, because as you point out, it would certainly be lighter than stainless. it is also true that aluminum has a much higher coefficient of thermal conductivity- which is one of the main reasons i chose NOT to use it. my feeling is that the shield should effectively be an insulator from the pipe, and stainless has a relatively low c.o.t.c. how fast the shield dissipates heat to the atmosphere is a function of exposed edge area- hence the large number of holes in the gaurd- and airflow. i notice that i often burn my leg while sitting or standing in awkard positions on the trail- when i'm not riding. you know, stopping for a bottleneck or some such drama in the bottom of a gully where there is NO airflow.
the other big reason for not using aluminum is really a manufacturing consideration. in order for an aluminum version to be as bulletproof as the stainless, it would literally have to be 3x as thick, and heat treated to a t-6 condition. the problem there is that as the thickness and hardness of aluminum increases, the minimum allowable inside bend radius also increases. because of space constraints, i needed the tightest possible bends for this design.
sorry this got so long, but since you asked, i thought i would give you guys an example of the kinds of trade-offs that have to be made during the product devlopment process.
wp.