I do not understand last question. I do not see any problem with mapping which rely on the item bounding rectangle.

You can have an infinite number of visually the same graphics items if you use different combination of position and bounding box parameters.

For example if you create QGraphicsEllipseItem using following rectangle/position pairs you will get the same result on the screen?
pair 1:
QRectF rect1( 0, 0, 1., 2 );
QPointF pos1(10.,20.);
pair 2:
QRectF rect1( 10,20, 1., 2 );
QPointF pos1(0.,0.);

Which way to define depends on your needs.