*groan* If it's metal it will effect rf wave propagation... invariably. Best not to have any and if any, as little as possible.
Without knowing the wave propagation patterns of the antennas there's no way to know it's effects. That's just for transmission, there's also the issue of receiving signals, ghosting and other obstacle induced phenomenon.
If it's using more battery in the case, it's definitely interfering with the antenna efficiency.
You are incorrecr. I don't think it will be productive for me to go into details of the physics in this instance, so feel free to believe what you like.