The wider the spillway the higher the volume of water required to be returned to the spa to achieve the spillway effect, not water dribbling down the face of the tile. The more water returned to the spa the less returned to the pool which depending on return location and number may decrease the "skimming" of debris. Normally proper pump sizing will overcome these issues.

So simply adding a larger pump will allow an increase size of the spillway? Our current plan calls for a 2hp whisperflow pump.

Yes, as long as there are enough spa returns to handle the increase in flow. Normally a variable speed pump would be used and speed can be adjusted as needed.

The larger pump must have the proper pipe size on both suction and return to deliver the spill you are looking for. A certain GPM is required per lineal foot to do so.

If your spillway is to have natural stone, when it's set, it should be pitched back towards the spa to create head and provide a good spill. An overhang of at least 2" will keep the water from dribbling onto the inside face of the pool.

Just "adding" a larger is only a part of the equation.