It's always been my theory if you're running a full exhaust, by the time it gets all the way down the head pipes, through the mufflers, over the axle and out the tailpipes it doesn't know whether you had headers or not.
Race car with open or no exhaust system, I'm sure headers are worth some hp, and have the scavenging effect to boot. I still hate to look under the hood of a nice mopar and see a bunch of crappy aftermarket parts.
