I have a scale now, the stock weights are definitely 20 grams.
My initial impression after changing to the 21 gram sliders is that the initial take off is a touch slower, but the engine sounds like it's working less hard at top speeds.
I think that if the engine is working less hard at the highest speed, the lighter sliders should have the same effect though. My understanding is that at the highest speed, the gear ratio will be at it's highest either way (assuming they're heavy enough to get there), the heavier ones will just get there sooner. So it wouldn't make sense to notice that the engine sounds quieter at top speed. Except that supposedly the shape allows both higher and lower ratios, which is why I wanted them.
I wish the bike had a tachometer so that I could see what's going on.
Perhaps some day I'll try using or mixing 19 gram sliders.