Laust: A Cabriolet is a convertible Golf. Part fro part, if the two cars were the same except for one was a convertible, the convertible would lose because of it's higher curb weight.
Fandu: If you want to bring in non-production cars, European Car has reviewed a crap load of A1s that even beat that New Beetle. But if you want to talk about the fastest VW you can buy RIGHT NOW, then that one wins hands down.
Emulex: I do believe that the Corrado was the fastest production VW to date.