I will redo the math again then using 18 mpg and 15.5 mpg.
I come up with a savings of $5376 using $4.00 gas and $6720 using $5.00 gas.
Using 19 mpg and 16.5 mpg.
Savings of $4785 using $4.00 gas and $5981 using $5.00 gas.
Sounds like a wash to me in the end. Pick the one you like best because the ownership cost are roughly the same. I know someone will now jump in and say that the direct injection and turbos will go out before it reaches 150,000 miles which is speculative. I think the maintenance cost will be about the same.
You are a stickler for details, throw in the 3.5% interest amortised over 10 years that I get in the account I mention, (and assuming that I pulled out all of my fuel expenditures from the account), and at the $4.00 gallon I quoted, I will be very close to that $1000 figure if not slightly over.
I did not mention the maintenance items that you did, but you are correct that if those turbos give out, or it would break an exhaust manifold bolt like Fords have been prone to, then I come out way ahead.