Figuring out what Photo Stream is uploading exactly
If Aperture's activity window isn't showing you the file name of what is being uploaded then my suggestion is a bit terminal intensive (you've been warned).

$ sudo fs_usage | grep 'DSC_'

you can replace what's in the ' ' with the beginning of your image names or even the suffix of .jpeg or .nef or what have you. You should even be able to write 'Aperture' just make sure that Aperture isn't in the foreground because that will give you automatic outputs from window server displaying the application being drawn.

This will output a list updated every millisecond of what files are being touched. So close all other programs you don't need and let Aperture try to do it's thing and terminal print it out. Fingers crossed!