Rollers or sliders alone don't really change the top MPH. It's more the RPM at any given speed they effect.
If they are too far out of range they will reduce mph.
Lightening rollers is the norm for gaining rpm, and getting more torque sooner. Same goes for the "fly" springs. More RPM for more torque.
Some variators add a few mPH just by design. They will allow the belt to travel further up the ramp plate, and = more speed. For the most part tho, the cvt simply controls the engine, vs, the engine controlling the cvt.
Bigger carbs/pipes/air box mods can increase power/rpm/mph. The cvt determines how it is delivered.
Think about it like swapping a trans or rear end in a car. You can install a 5.07 rear and pull wheelies but never get over 70mph. or go to a 2.70 rear gear, and crawl like a snail but top out at 120mph.
If you put in 3.83 gears you will go 120 but get there faster.
Install headers, and a carb you get 150mph.
Dial in your cvt to get the best balance of what YOU need. just lighter rollers will improve the feel of the bike. Better take off but slightly higher cruise rpm. A stiffer contra spring will improve the feel, and maintain the same rpm at cruise.
Adding both give you the most you can get from the CVT.