Well going from a 2.5 pinch to a 4'open collector I seen a 12 horsepower increase on the dyno.. which was a significant find more than any other single change that we made including from small carburetor to Big carburetor.. but the smaller collector accelerates the car so much faster it actually provides a better time slip. Now keep in mind this is on a fairly heavy two speed small block.. mph is definitely down with the smaller collector
A three speed would probably help the big collector accelerate as fast or faster than the pinch collector but...as you know it's hard to control all that low gear, especially if you are running leaf springs (which, IIRC you are) so at some point you have to do what you are doing.
As I've always said, if it makes more power in the dyno it SHOULD go faster in the car. Gearing, converter, suspension and shocks can all make an engine with more power go slower.
