It depends on what kind of engine you have. What they're usually "tuning" is removing the emmisions so it doesn't break down at the worst time.
Tractors or dozers usually have engines that are also used in other industrial applications, in other applications they might have a horsepower rating double that of the tractor/dozer rating. So, yes there are engines that can get an extra 100HP and better fuel economy. The tradeoff is obvious, an engine in an excavator at 100HP will have a good reputation, and the same engine in a fire truck or pleasure boat with 300 HP will be called the worst engine the company ever made. That tuning could be a software/chip type of thing, or on older stuff it's "turning up the screw" in a lot of cases (turning up the screw won't get you all the way there...)