While my good friend here at clubsnap bought a hoya +10 at a good price to reduce the MFD (min focus distance) of his camera, I got hold of a 25mm extension tube to do the same for my lenses. Here are what I tried (pardon any blurness, all shots handheld indoors):
24-135mm macro lens @ 135mm, no extension tube, distance = 0.4m (MFD)
24-135 @ 135mm, with extension tube, distance = 0.05m (MFD)
80-200 @ 100mm, with extension tube, distance = 0.5-0.6m (MFD)
80-200 @ 200mm, with extension tube, distance > 1m (MFD)
Pretty satisfied with the results. The 24-135 macro lens could focus at 5cm away at 135mm, that's quite enough for me. I did a rough check, with the ext tube on, the 24-135 lens could focus from 5cm to about 1m, quite a good range for insect macros. Couldn't get a high enough magnification with the 80-200 lens, but the lens will come in useful when coupled with the ext tube for farther working distances, like for easily perturbed insects.
Even tho can focus at 5cm, that's too close for most insect shots, so I tried a shot at a comfortable working distance of about 15cm. Here's it:
24-135 @ 135mm, with extension tube, distance = 0.15m
Botanic gardens, here I come tomorrow!