I understand where you are coming from, but don't expect to get better gas mileage w/ the turbo set-up. Just from a physics/thermodynamic standpoint, the fuel needed to generate 300 HP is essentially the same in either motor (though the torque curve is different over the power band, it will average out in a similar way). If you look at the amount of air and fuel needed to get x amount of HP, it will be similar whether its in a motor cylinder, or in a pot of coffee. Of course its not really that simple, as you get into flow efficiency, fuel atomization (and how much of the fuel you squirt in actually ends up burning), and a few other things.....but you can approximate things pretty well at least. If you are not "on it" all the time, your gas consumption would be better, but it really isnt that bad driving that way w/ a V8 either. I think the best thing you could do if you are concerned about gas mileage is do the rear end swap you were talking about, though it will hurt acceleration somewhat

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1999 C43 AMG ~~~ 744 on black, mostly stock
1991 190e 2.6 ~~~ m117 5.6 project car