I'm not sure about the Speed differences, but there are other things to be considered. Scaling things down does change the looks. Using the default FOV there is a certain amount of "perspective". When you scale down, this changes. Bigger things just tend to look bigger even though the world would be scaled proportionally. It's all in the perspective.
Other thing you should remember is z-depth. Now you are really using the camera range of min_value - 18000. That means the zdepth range is quite big. using a smaller camera range results in smaller z ranges. I just don't know which one is easier on the card as it has to sort the polygon z-order to know what parts should be drawn over others.
I do know that if your close camera range is too close and you are using long range, you might get some nasty z-tearing. (jaggedness in water that overlaps land for example) You can fix this quickly by setting a slightly larger close camera range.
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