I just did about an hours research on the sigma lens and the concensus seems to be that it is not a great lens. but if there was something like it that did get great clarity and speed I would go for it, unless it was $5000
Some other forum to that I can't remember. But I remember a lot of talk about it being "soft" and not holding focus. It also depended on the camera body. The Canon D40 does not seem to be compatible. I could not find any mention of it with the 5D Mark II that I have
Maybe it is what I am needing it for, landscape photography. From the reviews this seems to be more for sports, action and animals in nature. Read both Michael and let me know what you think.
I have not pursued the rental option. I can tell you from a long discussion on this forum that the Canon 1.4 tele is great (I have one) but the 2x is not so good. If you can find the discussion you should go thorough it.
Meanwhile, if you find a 50 to 500 or a 200 to 500 that is great and under $1000 please let me know.
I have rented lenses in the past, the best place I found was rentglass.com. They don't have a great selection but price and convenience can't be beat. My last rental was a canon 70-200 f/2.8 AF IS.