The whole point of a rear spoiler on a beetle is to reduce lift. They don't create any downforce, only reduce lift. This is a common misconception. So, it is a compromise between reducing lift and reducing drag, you can't do both.

Maybe Dave can tell us:

http://www.germanlook.com/Forums/showthread.php?t=5221

The Duck tail is still available from EMPI. Out of the box it fits like $#!* but if you do some fitting, cutting and build-up you can get it to fit well. I don't know how it performs at speed as I have not driven the car and havn't got any feedback from my son good or bad. He likes the look. I also cut a large opening in the top of the tail and installed a grill on it to force feed more cooling air into the engine bay.