Technically, 20% RPM is too slow even for 2000RPM fans, in some case, fans won't be able to sufficiently inhale fresh air and exhale warm air out especially where intake fans have multiple obstructions such as drive cages and clattered cables.
Now, with dedicated fan controller such as Thermaltake Commander FT, you'll have wider options of adjusting fan's RPM, again, its the fan's circuity that determine at what RPM not the controller.