The window in the piston is the most important part, if there was not window in the piston, there is no way for the intake charge in the crankcase to get to the "boost port" cut into the cylinder. Unless the boost port is cut all the way from the bottom of the cylinder, up through the bottom of the intake port, then out the top.
Even still, you will have better flow numbers with the window in the piston.
To sum it up: They both add a benefit. A reed valve conversion without either of them is pretty useless. If the "boost port" cut into the cylinder is done in a specific way it will provide its benefit without the need for the window (although it will work better with the window). If you just do the window in the piston (depending on the size and shape of the window) you will definitely see a performance gain with the reed valve conversion.
Reed Valve conversion without Piston Window or Boost Port = Meh
Reed Valve conversion with Piston Window and no Boost Port = Improved low end power, possible loss in top end (depending on window size/shape)
Reed valve conversion with Piston Window and Boost Port = Even more improved low end power, with more top end power too.
If you have any other question don't hesitate to PM me.
Sean Davis
Apollo Moto Racing
https://www.facebook.com/ApolloMotoRacing/