<p>The 2X converter effectively makes the 55mm f2.8 lens into a 110mm f5.6 lens. I believe you get the same working distance as you would using the lens alone, i.e. greater than with the lens and extension tube. One issue is that you are placing some extra (non-Nikkor) glass in the light path. You can expect some degradation of the image from that. The question is: Is the image better using the 2X TC or using the lens alone and cropping to get the same image size. I haven't tested this myself, but I have been reading about using TCs with telephoto lenses. The consensus seems to be that certain TCs, such as the TC-14E, don't degrade the image too much, while others such as the TC-17E do. Opinions seem to be split with the TC-20E, and there are different versions in that series. I am taking it as a given that you will get a sharper image with the ext. tube than the TC, assuming you can get enough light on the subject.</p>
<p>I have found that filling the frame and getting the highest close-up ratio isn't always the best solution when photographing insects with the very sharp Tamron 90mm f2.8 AF lens. By shooting at ca 1:2 vs 1:1, I got more depth of field, with plenty of detail in the cropped image. Of course, that also gives more distance to the subject, which is already longer than with a 50 or 55mm macro lens.</p>
<p>One other issue: I believe a non-AI lens or ext. tube, such as the M2 or PK-3, would damage the AI tab on most of the cameras we're using. Soon after the switch, some cameras came with a tab that folded up for use with a non-AI lens.</p>