Right now we seem to instantiate three relatively expensive texture-related objects

QString m_cachedTexturePath;
QColor m_cachedTextureColor;
QImage m_cachedTexture;

for all kind of PolygonGraphicsItems. However these textures get only rarely used (we have e.g. no textures for buildings, most areas, bathymetries, etc.). So I would suggest that we encapsulate all three texture objects into a wrapping TextureCache class which only gets referenced by a pointer. The actual TextureCache object would only get instantiated lazily when needed. This would probably save memory and instantiation time. A further optimization would take this approach further and would save much more memory (especially with regard to the image): we would reference each TextureCache object via an id that maps to the cached TexturePath (possibly also taking the cachedTextureColor into account). The TextureCache would get reference counted/cached in a dedicated cache container. As a result there would be only a single one TextureCache object instantiated for many AbstractGeoPolygonGraphicsItem-derived objects that use the same cachedTexturePath/cachedTexture-QImage.
This would again save memory and instantiation time.