With regards to the 7D, 1D4, FF, and cropping, there was a lengthy thread earlier with lots of sample images as it relates to capturing bird images. The pixel density of the 7D sensor is capable of capturing a lot of detail. Images require more work to clean up in post processing though.

7D AF is up to sports action. Used to use a pair of them at events. 5D3 AF system is superior as will the 1Dx be once you receive yours. Would suggest renting one before buying and trying it with existing lenses in a range of lighting situations and with different distances to subjects to see how you like the results of cropping, TCs, cropped sensors and various ISO settings in your final images. The test shots in Bryan's 7D review compared to the 5D2 would give you some idea of what to expect for image quality.