As far as the diffuser. I don't own a dellorto. But. Lets take the standard Puch Bing and use it as an example. A 15mm bing comes with a 2.17 diffuser. This is the inside diameter. Using a 2.20 will cause your needle to allow more fuel across the board. Which will effect your needle settings. The Puch bing does not have an idle jet so to speak. This is it. Larger equals richer, smaller equals leaner.
So say I have a Puch polini with a bing. I find that I really can't adjust the needle to a position I like. And or its lean at idle. I would increase the size of the diffuser to richen my idle. And since it richens all my needle settings also, I have to readjust my needle to compensate for the richness at part throttle.
Basically your just changing the relation of the orifice to your needle.