You can use a different throttle slides to achieve different venturi effects, among the hundred bazillion other things you can change inside the PHBG.
I've run a Polini with a 16:16 SHA. My current set-up is a 21 PHBG. The PHBG gives the bike more torque and a much smoother acceleration than the 16mm. All around better performance. The gearing has not changed, the pipe has not changed.
Minarellis are magically torquey stock. Try switching pipes or gearing if you are unhappy.