Will calling setContentSize() on a sprite in non-9-slice mode resize the sprite? At present this does not happen, but I do think it should - not doing so is unexpected behaviour.
How is PolygonInfo handled when switching between modes? Ideally we should be able to switch between the following modes without loss in data:
Quad Polygon (falls back to Quad if no PolygonInfo has been provided - manually or in the sprite frame) NineSlice
The default mode could depend on what initialisation function is used