Thanks for the additional tips, guys! Looks like the 70-200 f/2.8L IS
might just be the way to go...
As for switching to the 100-400 instead...hmmm... I'm hesitant to get it because it doesn't zoom 'out' as wide as the 70-200, but do you really think the latter would be pretty much useless at YSSY (I would definitely get the Canon 1.4X teleconverter - or maybe even 2.0X? - to complement)? Too many choices!!
Actually, could the 70-200 f/2.8L IS
be paired with a 2X teleconverter to equal the 100-400L IS
in quality/ability? What is the 70-200's max. aperture with the 2X teleconverter, at 400mm? Is the inability to us multi-point autofocus a major disadvantage to using the 2X converter with the 70-200 f/2.8 L IS
? Would the 70-200 f/2.8L IS
X teleconverter be good for YSSY spots (the tower mound, e.g.)?
Regarding a wider-angle lens to complement all of this: are there any decent/good lenses a bit cheaper than the 17-40?
Sorry, the above is quite a jumble, but I'm just trying to put everything out there. Thanks again to everyone for all the help!