I'll throw my two pennies worth in here ... unless you have irrefutable engine dyno #'s, then everything is theory !
Many theories have been tested and been held to be true, what works on one installation may not on another ( this holds true for fuel entry placement and throttle body design &c )
The radius on the trumpet bell mouth is a function of throat diameter ( there is a mathematical formula for this curve, but to be honest, it probably varies with air density &c, so flag it ! ) and should extend right to the backside of the horn ( as in the picture of the Honda B16 )
Change of Trumpet or port diameter is limited to around 4 deg before turbulence