Warmer air is less dense, which means you open the throttle wider for the same power. That means less pumping loss to pulling the air past the partly-closed throttle valve.
I think using a heater core would be counter-productive, as that will add a restriction to flow up in the air box. It's possible that it won't be enough to matter, but my first guess is that it would be. Testing would tell, and that's why it will be great to see the results from this!
-soD
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