alpha 1 drive extension

[NHGuy]

I saw an ad for a Donzi this spring, it had an extension of around 5 or 6" between the bellhousing and the Alpha drive. I forget what it is called and wanted to ask about it.
Looks like a way to get the propeller some smooth water on a fast boat. It uses a similar concept to the setback plates used on outboards.
Do any of you have any information or opinions on this piece?