There is one universal truth about engines and open exhaust. It will sound cool.
In terms of performance, there are many contributing factors, and not all engines respond the same way. Some will have improved performance and others will have decreased performance.
You will be decreasing the backpressure, and some of raw fuel (and fresh air) will be "scavenged" into the exhaust system. This reduces the amount of air and fuel in the cylinder at the time of combustion and causes the fuel to burn in the exhaust system instead of the combustion chamber.
You will lose some cylinder pressure and this will decrease your available torque, although your rpms may increase.
Your air/fuel mixtures will be leaner and you may burn your exhaust valve(s).
There will be a tendency of back-fire (or popping) during deceleration of the engine.
I know all the above to be true, but I still run an open header on my grandson's minichopper, 'cause it looks and sounds cool.