The adapter will reduce most of the benefits of a two barrel. Trying to neck down a bigger carb to a small intake just doesn't perform all that well. Also, you would need a progressive two barrel approximately twice the size of your one barrel since it would be only using half of it most of the time. Most likely what you have read is how good progressive's perform when running dual carb set ups. Here they work in pairs so the two primary barrels equals the original carb size, and the secondaries add slightly more cfm when opened.
Just running one would be way undersized so you would be into the secondary most of the time just to keep it running.
Jo
