well done mike.. i also don't think the difference to mkIII will be as big as difference between early generations and mkII.
money is a huge obstacle when choosing between L lenses, but if you're serious about astro you need to consider that the 17-40mm at f4 is great for star trails but not so good for night sky scenes. it's also quite a lot of overlap with your 24-105 (which i also use as a great all purpose lens).
there are some copycat primes that are not too bad.. or you could at least try adding 28mm f1.8 for astro. so many choices..
Phil
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