This has certainly generated plenty of discussion. Thanks for all the input everyone.
wayno - when you say the Arcteryx is more lightweight material, do you mean that as a negative? Would something like that be too lightweight for Tassie?
I don't tend to sweat a lot - for some reason my face goes very red instead

However, after becoming hypothermic once because I didn't think I needed rain pants (I had a goretex jacket on) I'm very cautious about becoming wet as I tend to get cold quite easily and don't warm up quickly. So I see the rain jacket as a way of keeping me warm, even if I'm wet inside.
I guess I'm at the point where I'll need to take a trip to Sydney and see what fits. This thread has helped narrow the options somewhat.
As for Pertex, I think I have a very thin cycling jacket made of Pertex. The seams aren't sealed but it's handy in case of unexpected showers or cold winds. Looks like it would tear easily tho.
The other place where I can potentially save a lot of weight is replacing my current sleeping bag... which probably calls for its own thread.