Have you had your head ported? 
Have you ever stuck a stock exhaust gasket in the port and actually looked at what is happening in there? 

Yup, the stock exhaust gasket often restricts the exhaust flow. 
Kinda silly paying the big bucks for a port job only to then restrict the exhaust flow, Eh?

47mm outside diameter, 41mm inside. 

Here's the fix, KWICK's Copper Exhaust gaskets. 
Made from 0.040" Copper on the 4000 watt Laser. 
Set of Four.