6T5 Cobra,
I was running 600 cfm Holleys with my 2x4 set up. The Carb shop I used did a great job on the build and the linking of the vacume secondarys. Of course it was mathmaticly over carbed so it ran a bit rich but it wasn't too bad. Throttle response was great and the car was fun to drive.
The 6 duce set up uses a progressive linkage so that at low throttle I am only using 2 of the 6. You do notice when the others come in
. each carb is about 275 cfm so at low throttle it's like running on one 600. The car is quick and has great performance on the street and hiway. I'm not a track guy so I don't have a point of reference there. It this point I do not see any reason I would go back to the 2x4.