velocity stacks work GREAT when you dont exceed a 1.0 pressure ratio. when you change the pressure ratio and the volume of air coming into the manifold, turbulence increases. the area around the floor of the plenum becomes stagnant because its high pressure and low/zero velocity and i think it changes the effective volume and decreases the surge tank effect that we desire in a manifold like this.
the bell-mouth/velocity stack contouring at the opening of the runners is highly desireable for laminar airflow and preventing boundary airflow from de-laminating... or whatev. but using velocity stacks to acheive that is less than desireable in a forced induction engine. bell mouth porting the ends of the runners where it meets the plenum would be a better effect.
btw, NICE welds!