Well if you don't believe your MB is running them at 100%, your next best bet will be a fan controller. But I think at Auto they are at 100%, as every board I've had in the last few years was like that with any form of fan control, auto=full speed. But maybe that was just the boards I chose. I dunno.
Research your fans to check and see what their max RPM's are, CFM etc, they may be maxed out and you just may be dissapointed.