It really sounds like with that tube the intake/exhaust flow is not getting the little flow stop shockwave at the right time. It is like changing the timing on an exhaust valve, these little engines use a shockwave of backpressure to stop the exhaust an instant before the piston closes the port and aids in combustion. Try a smaller diameter tube for about the last 3 or 4 inches, still get the great baffle out sound and maybe better WOT.If this timing is "out" it would be worse at WOT.