I always found this post very useful, so I`m going to add some tests that I made recently using Nikon D50, Sensor: Standard DX size (23.7 x 15.6 mm), Nikkor 105mm lens, Nikon 4T diopter and Kenko 2X Teleplus Pro 300 Teleconverter. Also I did some basic maths trying to understand myself the magnification provided with this combo. So here it is:
This is the 105 by itself:

I took this as a base pic to establish 1:1 magnification.
Then I add a 4T diopter:

As we can see it covers near 17mm, so magnification provided is 1.4:1
Adding a 2X to the 105mm we have this:

See that it covers close to 12mm, providing magnification of 2:1, prooving that the multiplier works equal for the focal lenght as for the magnification factor.
If we put this three together, we going to obtain this:

Covering 8mm I dare to say that magnification provided is 3:1
I also did some test with the Sigma 17-70mm macro:
Here`s the lens by itself:

Wich provides 1:2.3
And adding the 2X TC we get really close to 1:1

This is not intend to be a cientific post ,so I know I could use a tripod and other things to provide a more accurate result, but I think this help.