Hi Bruce,
To get back to your original question, it is normally not recommended to use teleconverters with shorter focal lengths. I believe it has to do with the curvature of the image that is projected on the sensor, which is stronger for wide angle lenses, but don't pin me down on that. Yet, on a 28-70mm zoom you may not be happy with TC results for the lower half of the range. Best try it out before buying.
A diopter will both increase the magnification and reduce the min focus distance, but it will also decrease the max focus disance. A +1 diopter has a 1000mm focal length which means you can't focus on anything more than 1m beyond the tip of the lens (that could be a good thing as it forces you to become better in getting closer to your subject). A +2 diopter, like the Canon 500D, has a 500mm focal length which means you can't focus on objects beyond 0.5m, which may limit you more than you want to for general shooting needs.
A diopter acts by reducing the effective focal length. For instance, a 100mm lens with a +2 diopter will behave as a 83mm lens. That's not all. For internally focusing lenses, focusing is done in part by reducing the focal length rather than increasing the physical length of the lens. For instance, the Canon 100mm macro lens gets 1:1 magnification at a focus distance of 31cm. Based on lens physics, 1:1 magnification means that the focus distance is exactly 4 times the focal length of the lens. So the 100mm lens has been tuned down to become a 310/4=77.5mm lens (this assumes there is a single optical element, instead of the dozen or so that are really present, so in reality it is probably a bit worse than the 77.5mm). Slap on the +2 diopter and the 100mm lens at closest focus now behaves like a 67mm lens. The good thing about diopters is they increase magnification and reduce working distance without decreasing the aperture of the lens, so an F2.8 lens is still a F2.8 lens after adding the diopter. (However, the nominal aperture is for focus set to infinity, the effective aperture is the nominal value time (1+M) where M is the magnification. So, magnifying more always causes loss of light and more challenges to focus.
An alternative is to use extension tubes. By actually making the lens barrel longer you gain magnification without losing focal length. An extra bonus is that you do it without adding extra glass elements in the light path that you need to pay for and that can degrade image quality. The problem is that the impact of extension tubes depends on the focal length of the lens with less effect for tele lenses. So on a zoom the effect may be too strong at the wide angle end and not strong enough at the tele end of the zoom range. To give you an idea, adding a 12mm extension to a 60mm macro lens gives you a 1.24x magnification, whereas for a 100mm macro it increases to 1.16x, and for the Canon 180mm macro the gain is only 1.1x.
Like diopters, extension tubes make it impossible to focus on objects that are too far away. For instance, adding a 12mm extension to a 60, 100, or 180mm Canon macro lens makes it impossible to focus beyond 43, 105 and 307cm, respectively.
Getting back to the teleconverter, you gain a full 40% of magnification, so 1.4x max for a macro lens, you similarly increase the apparent focal length of the lens and the working distance range is not changed, so you can't get closer than before but you are still able to focus to infinity. The drawback is light loss and some image degradation (minor with a good quality TC). A F2.8 lens will become a F4 lens. Finally, if the image projected by your primary lens does not have very high resolution (for instance because you closed the diaphragm too much) then the TC will magnify the image but you won't get any extra detail. You could just as well have made a 1.4x crop in photoshop and save yourself the hassle, cost, and lost depth-of-field, that any of the above supermacro tricks impose.
On my next trip (Red Sea in early June I hope) I want to play with extension tube combinations. The 12mm extension on my 100mm macro will give a working distance range of14-89cm in front of the lens and a magnification range of 0.12x to 1.16x, which I think is just fine. At 1x magnification the F2.8 lens behaves as a F5.6 lens, with the extension at 1.16x this becomes F6, which hopefully will still give somewhat decent autofocus, at least in shallow sunny shore diving conditions. My tube set also has 20mm and 36mm tubes so I can see how far I can, or want to, push this. (20mm working range becomes 13 to 60cm with a 1.27x magnification and 36mm give 12.5 to 38cm and 1.48x).
Finally, I have a Canon 135mm F2.0L lens for land use. Its normal closest focusing distance of 95cm isn't great for UW but it would be interesting to see if the fast aperture can be exploited with TC/extension tube combinations. With a 36mm extension I'll get 37-64cm working distance and 0.27x to 0.49x magnification with still an effective F3.0 lens at the max magnification. Even with an added 1.4x TC the effective aperture at closest focus would remain a very decent F4.2, especially considering it would have a 189mm focal length. My experience is that these things always work better on paper and turn out to require a lot of patience and frustration in practice but I think it could be interesting for shy fish or fish behavior where you don't want to disturb the scene and still get decent magnification.
Bart
PS all numbers are based on calculations and may be off a little bit.
