I am familiar why header wrap is supposed to work. Hot air is less dense than cool air and therefore encounters less restriction and thus higher velocity through the headers theoretically resulting in greater overall airflow through the engine.
Header temps are in the area of 1000 degrees and are individual tubes which have much more radiating surface area than that of a solid stock exhaust manifold.. A substantial difference which is why headers are a substantial source of additional heat. Most stock exhaust manifolds also have a heat shield that gets removed so yes, underhood temps with headers can get to be a problem.
Since the rest of the engine is closer to 230 degrees it doesn't really factor in here, its always the same temp with or without headers.
I ran wrapped headers on my Integra and easily noticed lower underhood temperatures and don't question their effectiveness. What I did question is whether the wrap contributed to the header welds that failed while wrapped. That question has never really been answered here either way but I was hoping someone had some hard evidence. Hopefully more info will come in.
Since I recently reinstalled my stock airbox, underhood temps are no longer the concern they were. BTW, the car runs better with the stock airbox with quicker throttle response and a little more torque mid to high rpms.
Thanks for all your responses.