The CDI and the magnet kit is for a clean set up as am tired of running the old jugad type set up that I have. As for the head mod and reed, well trying never hurts right?

Each engine's vacuum through different RPMs is known to the designer and manufacturer,and with this wisdom he selects the Carburetor size, jets ,and different settings...The Venturi of any carburetor decides the CFM and the speed(Velocity) at which the air will flow through the Venturi. For instance in a 150cc engine fitted with a 26mm carb if a 29mm carb is fitted there will be a drop in low and mid range because now the air speed is reduced for the same Vacuum,due to larger Venturi and the filling efficiency(Volumetric Efficiency) of the cylinder is lower than stock....but at higher rpms the venturi due to it's larger size will flow more,and aid in better performance....
So it is the air speed through the Carburettor Venturi that is referred to as Venturi Velocity, and has a direct effect on the engine's performance.

So in short, if the cylinder can not generate enough vacuum and burn more fuel than stock then there is no use putting a bigger venturi carb?

@PSR - Wonderfully explained & there's no better way to explain. Its like using a smaller dia hose on a running tap to generate more throw at the end OR choose larger dia hose on a running tap to get less throw.

Yes you are correct.

Yes .you are precise.

Well, then can we continue over to this thread, I've so many questions to ask & I trust you can help me with most of them.