Quick Answer: For Houston, the daily watt-hour output is increased, on average, by 30% with 2 axis tracking.
Caveat: But tracking has additional costs and maintenance, compared to a fixed array. Why not just install 30% more panels and forget tracking?
Trackers can maximize the energy harvested when the demand is high.Such as when the peaking generators are turned on.
Details: The National Renewable Energy Lab website has a calculator, in which one chooses the location of the array, and the method of tracking. Choose the location, and the tracker type (Fixed,1 axis tracking and 2 axis tracking)
Calgary(51 deg N. Latitude):_Baseline___+35%_____+55%
Conclusions: The real winner in terms of tracking is Calgary. In places of higher latitudes, such as Anchorage, the incident radiation is somewhat less.