We each have our own way of seeing and that influences the lenses we choose to use. For me, Iceland had many grand vistas of wide open spaces and I chose to use my 16-35 and 28-70 lenses much more often that I ordinarily do. That said, I went back to the LR2 statistics and to my surprise found that I only used these lenses for about 1/3 the shots taken. I used my 70-200 about 40% of the time, although I'd bet that most of those were at the wider end.
The rest were with my 400 with and without the extender. I shoot with a full frame body and found that for many of the puffin shots this was required to fill the frame. For other puffin shots the 70-200 was fine as the puffins are very tame and allowed me to get quite close. These shots include more of the background setting. The hardest part was dealing with the strong winds and making sure I wasn't blown off the cliffs. A little exaggeration but not that much. If you plan to use your 40D for birds / wildlife then your 70-200 with extender should do just fine. For a full frame body I would bring your 300 with the extender. [Edit - I also used my macro for tiny wildflower photography. That was one late afternoon when we decided to camp at a specific spot to wait for the sunset that never came.]
On the wide end I can only give you my experience, which is that I would have missed not having my 16-35.
I'm kind of slow in post-processing - still relatively new to digital and am trying to improve my LR2 skills. But I will post photos sooner or later, hopefully sooner.
Iceland is a beautiful country and the people are very friendly and helpful. Have a great trip and if there is anything specific you wish to ask, feel free to do so.