You can remove the Exhaust Valve which is usually done when replacing with an aftermarket system.
You will need to fit a plate to the controlling motor that the cables are attached to to enable the ECU to see both open and closed stops otherwise the EFI will pop a fault code, This in itself is not a worry but will mask any other fault that may occur and show via the EFI light.
The plate will come with the replacement exhaust in most cases as the manufacturers know it is needed.
As regards just removing the valve from the standard system and running without it, Besides having to stop up the resulting holes you will notice a loss low down but not having carried this mod out i am unsure just how much the bike will be effected.. I can see no discernable gain from that particular exercise
