Focal length is focal length, no matter what camera it was designed for.
The only difference between EF and EFs is that EFs lenses won't fit on your 1/5D body, and that the image circle is smaller (so even if it could mount, you'd get lovely black borders).
So, a 10mm lens will always be a 10mm lens. You could (theoretically) mount it on a 1/2" P&S sensor camera, or on a 4x5" Large Format camera, the focal length will still be 10mm (with even bigger black borders).
FOV Crop-factor is just that, Field Of View, it still doesn't change the focal length.
So, a 10mm lens on a 50D will look the same as if you mount a 10x1.6=16mm lens on a 5D.
If you mount a 20mm lens on a 50D, you'll get the same field-of-view as if you mount a 20x1.6=32mm lens on a 5D.
The "x1.6" is just an easy way for people moving between APS-C and 35mm to figure out what focal lengths they need to get the same Field of View as they're used to on the other system.