That's most likely just a manufacturing defect. You would hear complaints like crazy if it was an issue with all of the XPs and I'm sure there are plenty of guys running them here. I doubt there is an issue with the aluminized either.
Also, "back pressure" is a myth. But for the sake of this, the whole "back pressure" theory (which to be correct is about filling a volume for proper scavenging and flow) generally only applies to N/A engines.
"Back pressure" is bad for any engine.