For Cadet, TKM, X30, and Pro1:
PowerFinger Blade series: This series primarily improves acceleration. However, it can be a bit confusing because the very first level actually improves speed. But after that, the levels improve acceleration (and slightly decrease speed).
Firebird Falcon series: This is essentially the opposite of the PowerFinger - it is designed to improve speed. However, it can be a bit confusing because the very first level actually improves acceleration. But after that, the levels improve speed (and slightly decrease acceleration).