This is a subject that is really determined by the owner, the budget, and the terrain that will be ridden. Most will say do the cvt mods first while others say do the carb, exhaust first.
My take on it is do the cvt mods first, they will give performance and can be used with the stock exhaust and air box. The most popular are changing the rollers to a lighter weight for more bottom end power, along with the rollers the main spring will also need to be changed.
The first picture is a set of rollers, most 150cc engines will have around an 15-18 gram roller from the factory, most 250-300 will have 22-25 gram.
This picture is of a 1500 rpm main spring. A stiffer ( 1500, 2000 rpm )main spring will not move the driven pulley apart ( up shift ) as quick thus keeping the tranny in low gear longer. It also will close ( down shift ) quicker.
Along with those two items changing the clutch arm springs will raise the full engagement point of the clutch, alot like a stall speed converter in a race car.