Personally I don`t think going to 17-55 if you already have the 15-85 is worth it, I would miss the 2mm at the start and the extra reach.
Often even 2.8 isn`t bright enough and a flash will be needed.
But if you think that roughly doubling your shutter speed and the range is sufficient then its got to be worth it. I guess Australian sky's compared to Greenland sky may make the bigger aperture well worth it.
Just repeating what was said earlier, and its what I would most likely do if I was in your shoes>>>> keep the 15-85 and add the tamrom 17-50 ( non vc version) for about a third of the price. Its a quality lens, up there with the best bang for buck.
Canon Powershot S95, Canon 7D, 15-85, Canon 85 1.8 USM, Sigma 30 1.4, 50mm 1.8, Canon 100 2.8L IS Macro, Canon 70-300L IS USM+Kenko 1.4 Pro 300DGX, Canon 430EX II and RS 4 Classic