The Zetec is a strong motor and can handle a sustained 220, the stock thermostat is 195. Problem is, it is getting closer to the point of trully overheating, 250 is what I would consider overheating and ready to cause some engine damage. So your margin of error is less. To me, it's a risk assesment. "Do I want to risk the possibility of overheating for the X mpg gain?" Determine how much gain you have from the the mods that cause the high temps, then wiegh out wether you are willing to take the risk.
I'd be willing to take the risk for a couple mpg improvement, just be ready defrost
One indicator, for the Zetec, that you are too hot will be pinging. It will ping before engine damage occurs, that will let you know something is not right.
BTW, a manual swap with a Festiva fifth gear will proably pay itself off in mpg savings in about a year
