[GuidoK]

Still you dont know anything. material? thicknes? etc etc.
All you know and see is some dodgy pic and apparantly you can create a dynamic force model and flow model from that.
What do the sharp edges do in this case?
I also see sharp edges in the transition from the box to the itb runners in your example.

Having a sharp edge somewhere in the corner doesnt automatically mean lesser flow. For that we have CFD.

All you can conclude is that the volume of the plenum in the ebay box is bigger, and thats a good thing as it ensures less pressure drop when a cylinder opens. Thats the primary function of the plenum. Rule of thump the ratio has to be 1,5 or bigger. The ebay box probably has that with ease, your first example... not so sure. Do you know what plenum ratio that has?