I had a Holley 870 Avenger vacuum secondary on my Roush 427R at one point. I thought it was ok to do as the Superformance dealer was putting them on many of the Roush engines they were installing. Boy, is that wrong. I took quite a bit of time to make adjustments using all springs options and assorted jets. The graph below represents a casual start in 1st gear, casual shift to 2nd gear, then nailing it to test response time.(It is not an acceleration test) It was very frustrating having 1 to 2 seconds of delay at the punch. I did many, many passes, but the three in the graph are about as good as it got. A simple call to Holley tech may confirm that a vacuum secondary carb is not the best application for a high HP engine in a 2400lb manual transmission car.
I learned quite a bit over the next couple years after getting my first Cobra and I'm sure I have much, much more to learn, but looking back at the vacuum secondary ordeal I wish I'd of just tossed the Avenger in the junk heap and bought the mechanical secondary Pro-Systems carb from the get go. In my opinion just do it yourself and get it done.
The drag video in my signature is a start from idle with a Pro-Systems carb.