The dyno results I have seen so far all show improvements over the standard system.... so throughout the torque and power curves with no valve.
It will affect the fuel mapping if you remove the valve and I presume you are better off having some remapping done with a slip-on and full system.
I have also heard an argument that the valve is a restriction at higher rpm... but I don't know.
Put it this way, I will sacrifice a tiny bit of very low power (1-2hp below 5000) to gain up to 12hp at peak (if you believe the results I have heard of with the Akra system).
I was originally very protective of the exhaust valve and was looking at getting the muffler repacked just to keep it. But, once I heard the "alleged" gains from the akra system... bye bye valve
