I currently use a 70-200 2.8 IS II + 2xIII with my 7D when the reach is required. This is maybe 10% of the time I use that lens at most and is why I went that route. When I do need the equivalent 640mm, it is for shooting my 4 children all involved in water sports (human powered, so focus speed not as much of an issue). With that range I still need to crop some times, but as I'm a hobbiest the long range superteles are not justified, and stand out to my wife as obviously being very expensive. Once the kids are at a more competitive level I may rent once in a while for fun.
I would love to upgrade to a 5DIII this summer, but the 600mm focal length gets very expensive in FF, with the exception of the 300 f4 when I already have the 2xIII. Virtually everyone on here seems to say once you go 5DIII or 1DX the 7D collects dust. I could not justify keeping it and would sell it and the only EF-S lens, the 10-22.
Now lots of people love the 300 f4, but it gets bashed when discussing using it with 2x. However, I think the bashing isn't completely justified, it is being compared to more expensive lenses. When I compare the ISO 12233 against the Sigma 120-300 2.8 that is 3 times the price, the Sigma is only comparable in the center with or without converters, for example.
On the ISO 12233 charts the 24-105 can be compared between a 60D and FF, the lenses in question can not. Obviously it is not as sharp on a 60D/7D sensor.
So the question is, even though the 70-200 combination will get the best image to the image circle, will capturing that image on the 7D sensor degrade it enough to be about equal to the 300 f4 combination image captured on the 5DIII sensor. Equal image at almost equal focal distance, and hence still relatively equal cropping ability.
I'm hoping to benifit from the obvious upgrade for 90% of use without loosing quality at 600 mm with the suggested option.
WOW, that took way more words than I thought it would!