Ok so is this RPM related at all or just 5th gear or what? Go out and test it. Get to 4k in any (every) gear and floor it. You should have no problems and go right to redline. I can not tell you how it will feel under 4k because I never drove a car with wired aux. ports but know they do lose a little low end. Just tell us exactly what it does when you do this. And just so you know, most NAs don't have much in 5th to begin with.

Now, could it be ignition related? Possibly bad plugs/wires? Maybe one wire isn't all the way on? A coil not firing?

So the concern you have is simply an extraordinary lack of power, right?

If it isn't anything to do with the intake or exhaust being clogged, it pretty much has to be a fuel, ignition, or compression issue.