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@Airhead
Yes, you do seem to be right about the perspective compression. It is a product of the distance from the subject and you will simply be cropping the picture. It's slightly counter intuitive because you are comparing your '40'mm lens and true 40mm lens not the 20 and 40.
However I'm sure depth of field increases with wide lenses and that doesn't change with crop factor. Tap in the 20 and 40 mm lengths to this calculator and you can see the different dof.
http://www.dofmaster.com/dofjs.html
A fast 20mm comparing with a 40/2 or a 50/1.4, not really. Fast ultra wides are usually huge too. Horses for courses though.