Exhaust crossovers are made to aid in exhaust scavenging. The exhaust rushing through the LH header will essentially pull a vacuum on the crossovers (like a carburetor pulls gasoline) and help exhaust flow out of the RH header and vice versa
They probably do give a little bit of performance at high revs, but I'm inclined to trust Monte's "butt dyno" that you won't notice much, if any performance difference. They also essentially hook up each cylinder to two mufflers instead of one, quieting the exhaust.