I only know Rotax, but I’ve never found them to be spectacular below, oh, 7k RPM. Of course, all I’ve ever tweaked has been the main jet.
The needle clip governs how high the needle sits in the carb. There are multiple grooves the clip can sit in on the needle… The higher the groove, the leaner you are at partial throttle. At wide-open throttle, the needle is out of the picture regardless of clip setting, so it doesn’t really do anything for “low-end power.” The Rotax manual actually explains it pretty well (but then again, I’m a Mechanical Engineer, so take that with a grain of salt :loony: ).
Conventional wisdom is to leave the clip in the highest groove, maybe 2nd highest, but otherwise it impacts operation only occasionally (how long do you keep it at 1/3 throttle?).