Bottom line most of the time is quieter = less power, and less power capability.
As far as I/H/E not being worth it, that is bullshit.
for the money, they produce a noticable change in the cars low end power/torque, acceleration, and high RPM function. Depending on what headers you buy, they can do different things also. You can't tell me that your going to get any sort of performance capability from stock exaust that was build for super-low emissions and a tire-melting 102 hp.