O.K. you got me[paranoid] Of course VW had to have a funky set-up in their history[

] . With the Z-28 the plates are the same size and open at the same time, my Isuzu trooper is set up like that as well. It is dual butterfly, but it is not a progressive set-up.
That engines need back pressure is a myth. What they need is exhaust gas velocity to produce a scavenging effect in the cylinderhead, drawing out the burnt gasses. If you put too large of dia. tubing in your exhaust, you lose tq. due to the lack of proper velocity, not lack of backpressure. Putting a muffler, or any other back pressure creating device, helps create a scavenging effect in an improperly sized exhaust system. In street systems it is a trade off. You have to have a muffler on the street, so you need to run larger dia tubing to compensate for the increase in back pressure. If you remove that muffler and retain the large dia tubing you will lose low end tq. and some hp, leading many to believe you "need" some backpressure in the system.
Those tapered headers take a large volume of exhaust gas intially, then taper the tubing size to promote higher gas velocity, which in turn helps draw exhaust out of the system. Take a look at any racing series you want, unless mufflers are mandated by the sanctioning body, you won't find a race car with any restrictions in the exhaust.
The T.B. is serving a different function, it restricts how much air the engine can pull in, it is possible to go too large. You would have to size the T.B. to the point where it's size exceeded the engines ability to draw air at the proper velocity. Going to a smaller size is in effect choking the engine off. The vacuum created by the engine pulls the air in, on the exhaust side there is nothing helping the exhaust escape other than the exhaust stroke of the piston, so the correct exhaust sizing and design is required to help draw the exhaust out of the cylinder.