You may see a 100f temp drop with the Evo Spacers but if you do it will be on the surface temp of the surge tanks and not intake temps. I would be surprised if these actually generate any HP gains.

The Bimmer guy is not the first person to complain about the shop, their work, their attitudes, and their products. To be fair they are far from the only shop with complaints.

Without being able to see the pictures of the guys header there is no way to tell if the rust he was complaining about is normal surface rust or something more serious.