taper can increase velocity and or smooth the incomming air.
on a tb i doubt it gives any noticeable increase , unless of course u have a tb that is horribly designed
when it comes to replacing a stock tb with a larger or bored out unit most ppl think they are feeling a hp increase , when in reality if u take the stock tb at 1/4 throttle opening and then the larger unit at the same throttle opening the larger tb will alow more air into the engine.
so u dont have to step on the gas as far to accelerate like u did with the small stock tb
go to large with a tb and the throttle becomes more of an off on switch