The MTF value (resolution) drops to zero at the diffraction limit, ie is a brick wall cut off, and above that increases progressively with increasingly widening aperture until eventually it is not diffraction limited. Here is a page from textbook with a computed graph
https://spie.org/publications/tt52_151_diffraction_mtf?SSO=1 and the equations.
When you read articles that say the diffraction limit is not a brick wall they are oversimplifying and what they mean is that there is progressive loss of resolution as the aperture is decreased until you hit the diffraction limit. That is, you cannot resolve details that are smaller than the diffraction limited but larger ones get progressively sharper with increasing size. This has been a classic problem facing astronomers and microscopists throughout the generations. Once you hit the diffraction limited aperture, increasing the focal length of a telescope, for example, without increasing the size of the aperture does not increase the cut-off resolution dictated by the diffraction of light.
The above applies to a purely diffraction limited system. But, there are other factors such as aberration of the lens, Baeyer, AA-filter etc that will lower the diffraction-limited MTF still further. Oversampling, change of strength of filter and the effect of the TC on aberration might help to some extent.The take home message is that putting a 2xTC on a predominantly diffraction-limited system will double the size of an image but won't do much to the minimum size of an object that can be resolved. You would do just about as well by up-resolving in Photoshop.