No, I don't think I misunderstood at all.
If there is unburnt FUEL, not just oil, it's too rich. The only time I have signifigant amounts of unburnt fuel in the exhaust is during the first two or three tanks of break-in. If there is fuel in the exhaust that can get back inside the engine it can cause harm.
As for the header, adding, or removing, as little as 1/4" can make a difference in power band and performance. If he's thinking of adding copper pipe then he's thinking about adding, what, an inch or more? That will absolutely change the powerband and performance. Will he be able to tell? With that engine, his tuning and his unknown experience level, I cannot say, but change it, it most certainly will.
And I do understand the frustration with the oil on the side but it's really just the nature of the beast. Changing where the pipe exits will only change the part of the body that gets oily.
And lastly, I was not discouraging him from trying, simply telling him some things he may not know.