I found in my drawer a kit of close up add-on filters, manufactured by Cozo. It consists of 3 filters:+1, +2 and +4, so one can have a close up from +1 up to +7.
I wanted to see how they work (or don't work) with my 50mm/2.8 Sony Macro lens.
I started by taking a close-up photo of an handkerchief. Here is a crop from the original photo
Then, I put a +4 Close up lens, and tried to take a photo. The AF didn't work - I seems it doesn't like adding optics to the lens, so I had to tune manually. Here is the result:
No change in size, as I had to tune back the lens, so eventually the distance of the tip of the lens from the object remained the same. The only difference is that the photo is somewhat darker due to the addition of the filter.
Next, I tried to see if I can extend the maximal magnification of this Macro lens using this close up filters. Here I tuned manually the lens to the closest point it focuses (1:1)
and after adding a +4 close up filter (I kept the same lens settings) I received the following photo:
Well, here we have a change, I succeeded in magnifying the image by ~30% by using the +4 close up lens. Does it worth the investment - I doubt it does, you can always crop the photo to get a larger image.
To conclude - If you have a high quality Macro lens - adding close up filter will not help you :(
PS: I found this article after getting some feedback: