[elhigh]

1) Upgrade the head bolts. That's a known problem with those engines.

2) Absolutely do anything for the aero. I think the belly pan is the better way to go but if you want to start saving money immediately the air dam will start paying dividends right away.

3) Aero reduces demands on power, and that engine never had any power to spare. It'll feel a lot better on the road with the aero treatment.

4) I think 80mpg might be achievable but only under a very specific usage envelope of steady, modest speeds on the flat. I remember Popular Science reported about 70mpg at 45mph in a Rabbit Diesel; the Caddy's longer profile will put it in a better aerodynamic position if you add the kammback aerolid.