Curious to your results as well. I have been told annulars don't work as well as downlegs in a 4150. The venturi is just too small.
IMO, booster selection can be somewhat subjective. Put downleg boosters in a "big" 4150 venturi, say 1.58+, and in some (many?) cases it's going to suffer from low booster signal and ragged fuel distribution at low(er) RPM.
A "big" venturi 4150 carb with annular boosters may not flow any more than a smaller venturi + downleg combination, but it may improve atomization and fuel distribution for the same flow rate. Whether that translates into a performance improvement is tied to the combination.
I also have a hypothesis that two annular boosters of comparable size and dry-flow capabilities may result in significantly different wet-flow results where the booster with improved atomization actually reduces the total air-fuel volume significantly. If that's true and the configuration w/ the better atomization -- yet lower wet-flow rate -- improves performance, then that could (would?) reflect an air-fuel mixture quality vs quantity scenario.
Top view of the 1.61" v annular carb pictured above; that's a big hole to park a typical downleg in.
