Macro lenses have 2 important features, being able to focus closely, and having a flat focus plane. With extension tubes, you can solve the 1st problem so you can focus closely. However you can never have a flat focus plane (blurry borders, edges etc) with any method as the lens, itself, needs to be designed that way. Also stopping down to f/11 will make you loose resolution and it is very close to diffraction. Usually optimum resolution are revealed once the lens has bene stopped down 1 to 2 stops.