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Hi Doug,
FWIW:
In many cases, most Model A owners have lots of respect for "all" of the "different" Model T/A/B opinions of others, mainly because these opinions are for the most part based on "real" past experiences.
And, almost every Model A/B question just never has one (1) cut and dry response.
I think that after 80+ years or so, some could possibly find a B carburetor on E-bay that someone found under his neighbors hen house, buy it, and wonder why they get 10 MPG less than the A carburetor someone found on an abandon turkey farm under an out-house.
Intake size as mentioned often can affect performance, "especially" at higher speeds when one can hear the carburetor intake whistling.
I used to talk to Mr. Charlie Yapp often years ago, and in his "Journal of Secrets of Speed Society", he listed very interesting and informative articles written by Henry Miller, Jim Brierly, Murray Fahnestock, Paul Moller, Bob Rentz, Ron Kelley, Urb Stair, and several Model A "improvement" articles from the 1930's Popular Mechanics and Popular Science.
Mr. Yapp also wrote this years ago:
"When Ford redesigned their four cylinder engine, for the third time , (T, A & B), one of many changes incorporated to increase horsepower was the new Zenith updraft carburetor. This cast iron carb has
larger air intake, and exit ports, a larger venture, and
most importantly, a power jet that kicks in at higher speed."
The B has an intake of 1-1/8" diameter in lieu of the A's 1" diameter ..... a mere 1/8" difference .... but wait ...... what happens when Mr. Pi x and Mr. R-square attacks this 1/8" ..... i.e., 3.14 x .5 x .5 = .785 sq. ins. for the A intake; vs 3.14 x .563 x .563 = .995 sq. ins for the B intake; where .995/.785 = over a 26% increase in a B carburetor's intake's "area" for the engine's fuel mixture flow.
Existing conditions of 80+ year old A/B carburetors mixed with repro and original jets can also result in many different experiences and many varying opinions.
At higher speeds, a properly re-built B ...... or an original B right out of the box should perform better.