They can really improve mid-range torque because the intake charge can not flow back into the carb.
It does add a slight resistance to air flow and can impact top-end power but generally not by much. Kind of a trade off.
Note: Cheap reeds can break off, be injested by the engine and cause considerable damage to the piston & bore. Carbon fiber reeds would just pass through I guess but stainless steel reeds are killers.