|Generally speaking, hard wheels are for smoother surfaces, softer wheels are for rougher surfaces. Ramps and bowls are usually metal, wood or finished concrete so a hardness in the 90's and up would give more speed (goes the same for speed skating). Whether you're inside or outside while you're skating is irrelevant. Below 90 hardness gives more grip and flexibility for handling pavement.
If your wheels are still decent putting in new bearings will give the most noticeable change.