Had the Derbi out again last night. Oldest boy (Ninjette rider, 6', 140#) aboard. He loves the thing. Wishes it had a bit more acceleration, but once it's above 40 or so it's not bad at all. We were running 75 a few times and he said there was plenty left. The taller gearing (+1T countershaft) helps on that end, but is a slight disadvantage off the line.
Made some slight adjustments to the idle mixture screw and idle speed to get it to 1500. Idles smooth and pulls clean all the way. I've found that adjusting the idle mixture screw to get the highest RPM, then going richer until you hear a slight drop in RPM gets you a good idle and a nice transition off-idle. It's giving you a slightly rich setting that keeps the transition smooth. After that's set, turn the idle speed screw to get the speed you want.
That's the system I use to set the idle mixture on almost any carbed engine as long as the jetting is correct.