Eric,
The 5 HP increase came at the top end of the power band. The performance increase is proportional to speed and how much backpressure exists in the system at any given speed. In general, the more backpressure you have the more you will benefit from having a crossover pipe. That is why most race cars (e.g. drag cars) with open headers don't need them, but superspeedway cars that have to run a longer system of pipes to get the exhaust past the drivers compartment and out to the outboard side of the track will benefit from them, plus they help in scavenging in that case.
