Or you could save up $630,000 and get the 918. 78 MPG!

I have a feeling that if I could save up 630.000$ on a whim... last thing I would worry would be MPG..

Think Top up Vs Down affects mileage?

From a technical standpoint? Yes, the aerodynamic drag increases, so your car has to "work harder" to cut through the air.
Is it noticeably different? I would doubt it, maybe by .5-1 mpg difference.

On my first fill up I achieved a 13.5 MPG. This scared me because I drove extremely carefully and did not rev the car beyond 3000 RPM (was still learning how to drive stick).

I think it was a miscalculation, but I'd like to think it was true just for fun. I get around 16-18 MPG, and interestingly it doesn't change much even if I try to drive conservatively or boot it all the time.