We had a 440 with all the usual, ported iron heads, torquer2, .509 cam. 10" B and M converter 3.91 gears etc. We switched the 750 d/p to a 850 d/p and dropped a solid 2 10th's (12.50's - 12.70's). We tried the usual jet, timing adjustments and just couldn't get it back. I'd sold the 750 and wished I hadn't. About a year later a friend was selling a couple of 950's he'd had on a T/R BBC. I asked if I could try one and said if I went faster I'd buy it. We slapped it on, set the idle and picked up 3 10ths off the get go (12.40's) Later that same day we ran a 12.30 and eventually, we got that combo to go 12.00's.
Go figure. Obviously I bought it but I scratched my head as to why we picked up over both the 750 and 850. I found out that the 950 wasn't really a 950cfm rated carb as it had smaller venturies than the 850, it just worked right for that combo. My experience.