I find both to be useful tools for changing track conditions or even temporarily masking a failed setup attempt.
If you often turn from lock to lock, like on a tight or fast track, slower servo speed can really help to get you a smoother transition.
If you are constantly turning in small amounts then expo is a better adjustment and can really tailor the steering curve right where you need it.

Of course neither of these adjustments will reduce total steering at full throw, 1 or 2 percentage points on the dual rate can go a long ways at making corners easier, but could get you in trouble in tight sections.