I just put on some brembo cross drilled rotors. I noticed a black metal guard behind the disc. What is it there for? It seems that it would hamper the flow of air around the disc.

Would your brakes cool quicker without it?

Can they be removed easily?

 

It's there to stop random pebbles and debris getting stuck between your rotor and caliper, and in general to keep stuff out. You can remove it, in fact most if not all BBKs require you to remove it, but it won't really give you much performancewise, and you're more likely to get stuff stuck between your caliper and rotor and possibly scar your rotor.