I just installed Dr. Pulley sliders (20x15 16 gram, a set of 6 sliders) in my Kymco Yager GT 200i. Stock rollers were 16.5 grams. I notice the RPMs at low speed are a tiny bit higher, and at high speeds, they're about the same. To ensure the sliders slid easily, I put a thin film of high-temperature synthetic grease in the channels where the sliders ride... the grease is rated to not melt until it hits 600F, so it shouldn't liquefy and fling out. It's usually used for greasing disc brake pistons, I found it at Kragen Auto Parts.
I'm going to fill the centers of the sliders in with epoxy to give them a bit more weight, hopefully it'll lower my in-town cruising RPMs by 1000 RPM or so. If epoxy doesn't do the trick, I'll drill a hole through the epoxy on the drill press, lathe up a set of brass weights, press them into the drilled holes with the Dake press, then epoxy to secure it. And if that still isn't heavy enough, I'll go with a new set of sliders, with an even heavier brass weight... rinse and repeat until I get it tuned just right. But I'm pretty close to perfect right now... just need the in-town cruising RPMs dropped a bit.
16 grams is the heaviest slider Dr. Pulley makes for this scooter... I wish they'd make heavier, but there you go.