Hi GTom, i had my SA GTi with a WO Zenithstar 73, a Nikon D5300 and a SvBony guide scope and camera, total weight of around 4 kg plus i had to put an extra 500 g counterweight in addition to the 2,3 kg one, which comes with the GTi, in order to reach balance in the ra axis. I was able to get 180 sec exposures, but had to throw away about 20-25% of them, due to outbreaks in guiding (if you are interested, i could provide my guiding logs from phd2), my resolution was around 1,82"/px, guiding was on good nights around 1,3-1,4"/px total rms (i had mine hypertuned, aka new good ball bearings and good grease with graphite in it) stock it was guiding at around 1,6"/px, which is really good btw, compared to other GTis i have seen. Although i was able to get good final pictures with it, it wasn't consistent enough for me, so i upgraded to HEQ 5 pro eventually.
One other thing i should mention, i had mine also mounted on a carbon fiber tripod, although light in weight, it was extremely susceptible to wind, even a light gust would influence guiding.
I would say up to 200mm focal length objectives, lenses or small telescopes, the GTi can handle it just fine, if you are mounting close to the weight limit or even exceeding it, i wouldn't recommend it, unless you are happy with short exposures like 30-60 sec maybe or to throw out a good portion of your exposures due to tracking inconsistencies. Canon 6D with 600mm focal length would give you 2,23"/px resolution, so it would check out in theory, but practically, packing 6 kg on that little guy, is overkill.
Clear skies !