@Rick drla specialist- I have asked this question before on other forums as I am not a believer that the Dellorto is some new whizz-bang revolutionary concept, but a slightly better version of an already proven and capable design. I've looked but never seen any direct flow bench comparisons (same flow bench, same day) and no one (that I know of or read about) has ever tested them back to back on either the dyno or race track, so I'm led to believe that the flow improvement (and hp increase) is not big enough to be felt by most enthusiasts on the street. Between my (years and years and years ago) dabbling at a little bit of bracket racing and riding in friends VW's with similarly equipped but Dellorto'd engines I didn't see enough to make me convert and sell my fairly large collection of Weber jets and emulsion tubes.
The exception to all this is some (early?) versions of the 40 IDF- the carb tops are machined such that they will only accept 32 mm venturis, whereas later versions and all 40 mm Dellortos will take 34's. I am of the understanding that those earlier carbs can be machined to the later spec and use the bigger vents.
So if you are of the first camp (the Del is a whizz-bang revolution) then we'll just have to agree to disagree. But that's ok- it makes life interesting... Al