I haven't used one, but I might be looking for one in the future. I'd like to see the response to this question as well.
The Weber (or Holley) progressive has a bunch of problems of its own, again attributable to an intake manifold that's just too damn big. There've been some attempts to improve that using a non-plenum design (helps driveability a little, loses top-end power) and one which incorporates a giant preheat system that runs up around the plenum: http://aircooled.net/new-bin/viewproductdetail.php?keyword2=FSK0028&cartid=0325200594136636
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