It was a T/Pipe bung, as are most Dyno setups. I don't think that was a big factor, as we also ran a test day on the same dyno with about 10 other cars. Most were EFI, and my curve was far flatter than any other cars. Some of the aftermarket EFI AFR curves were all over the place ...
The tunable Vacuum secondary also helped smooth things out. The Vacuum secondary carb was really good in Autocross. Very smooth power delivery.
I later built a double pumper with the same specs and parts. That was much more difficult to drive in Autocross, but proved to be slightly faster on Road Course and Hill climbs. In autocross you would just haze the tires if you got into the Secondaries too quick.
Hill Climbs with weight transfer to rear tires and Road Races with higher cornering speeds, reduced the tendency to spin the tires on corner exit. So you could jump on the power quicker and harder.