OK a couple of items:
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Yes, siping increases rolling resistance but not enough to worry about.
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Read here about OE tires: http://www.barrystiretech.com/oetires.html
Short version: OE tires are designed to the vehicle manufacturers specs - and typically those emphasize fuel economy - and they get that by sacrificing treadwear and/or traction.
ALL - let me say that again - ALL tires that come on new vehicles are specially made to the vehicle manufacturers specs - no exceptions. They are NOT like the tire that you usually get from the local tire shop - except to say, you can buy the OE tire there.