#1 - 1:1 magnification is .. well 1:1. The only difference is the minimal focusing distance due to the small differences in focal length between the three. In real life you'll not notice it.
#2 - I've seen a lot of very impressive macro shots done with both the Tamron and the Pentax. There really is no noticeable difference between them in optical performance or field of view.
Regarding the image you linked to, you'll have to notice two important factors: First of all it's shot with an Olympus that has a smaller sensor that will give you a smaller crop straight out of the camera. Second it's tagged with EX-25, in other words the Olympus EX-25 macro extension ring and thereby you'll have a greater magnification than 1:1 My guess is that you're being extra "amazed" by the larger magnification of this combo.
#3 - As far as I know the three lenses (and their various versions) have no differences when it comes down to compatibility with teleconverters or extension rings. The major difference will be in the size of the filter thread. That can be something to consider of you'll want to share filters with other lenses or use a ring flash with at particular thread. That said I'm using my Tamron with a 55 to 49mm step down ring for my ring flash.
A few extra notes from me:
- macro lenses have a long focus throw. If you're going to use autofocus, a focus limiter function on the lens can be quite handy. (but autofocusing is often pretty useless at 1:1)
- on a DSLR crop usually gives better results than teleconverters
- at high magnification you'll need a flash to get enough light, even outdoors
- the Raynox DCR-250 is used by some as a cheap way (with great success) to achieve extreme macro combined with a 1:1 lens, there are lots of examples @ flickr
As mentioned I've got the Tamron SP 90, an older version (pre DI). There are a few examples from it at my flickr account if you're interested. Have a look under the
"small creatures" set or the
"macro" tag. I use the DS a lot for macro as it supports the oldskool TTL mode of my AF080C flash.