There are several companies that offer reprogramming of your factory ECU. For a fee, they will take your ECU and (supposedly) change the code for a variety of purposes.
Some companies claim to add horsepower by changing the ECU parameters. So far, we have no evidence that any of these changes work for the turbo Eclipse. Here's why:
Turbo boost is controlled by a vacuum feedback system. While the computer does have the ability to reduce this boost using the boost control solenoid, it cannot increase the boost beyond the factory setting. Any ECU programmer that claims "more boost" from a chip swap doesn't know what he is talking about.
The spark timing on the car is optimized dynamically by using the knock sensor and always advancing the spark to just before the onset of knock. For this reason, any programmed-in change to spark timing will be overridden by the computer when it dynamically adjusts the spark.
Fuel delivery happens in two modes. In "closed loop" mode, the computer reads the oxygen sensor to try to get the perfect 14:1 air/fuel ration in the cylinder. At cruise, your car operates in this mode. Since this is again a feedback type of operation, any changes in the base fuel settings for this mode will be overridden as the computer seeks the 14:1 ratio.
"Open loop" mode is the only place where fuel settings can affect power. At full throttle, the computer does NOT read the oxygen sensor, but instead just looks up a table of air flow vs. rpm, and adds fuel according to the value found in the table. A careful tuner could change the fuel mixture to increase power, since the factory settings are conservative, especially at high boost levels, where extra fuel is added to the cylinders for cooling. Leaning out the air/fuel mixture can increase horsepower in these conditions.
That said, the ECU only has a fixed map, and unless you changed the ECU every time you modified your car, it would only be optimized for a single set conditions. So unless you have a bone-stock car and intend to keep it that way, or unless you've had the ECU custom-programmed for your car, WITH THE CAR ON A DYNO, beware of claims of computer swaps that add horsepower. Either the increase will only be ideal for a stock car, or it will be only work for a given set of modifications.
The solution for this problem is a programmable fuel computer, like the A'PEXi Super AFC. It's no more expensive than a custom ECU, and it's much more flexible and effective.
There is one company that doesn't claim to add horsepower, but they do deliver genuine useful improvements by reprogramming ECUs. Technomotive, of Santa Barbara, CA, offers features like removal of the "fuel cut" feature of the ECU, custom rev limiters, removal of the top speed limiter, automatic shut-off of the a/c under race conditions, and more. Another favorite modification from Technomotive is the "stutterbox" feature, which allows you to sit at the drag race starting line with your foot on the floor, with the revs holding at 4500 rpm (or whatever rpm you want), so you don't have to watch the tach and the tree at the same time.
Note that only some ECUs are eligible to have this done. You must have an EPROM ECU if you have a 90-94 car. The EPROM allows reprogramming. If your car is a 91-94, look at the sticker on the outside; if yours has an "E", it has the EPROM. For 90, you must open up the ECU to find out. All Galant VR-4s have EPROMs.
Some of the 95-96s have the capability to be reprogrammed (see the Technomotive web site for instructions on figure out how). 97-99 ECUs have flash ROMs which are (so far) not reprogrammable by Technomotive.
If it sounds like we're endorsing Technomotive, we are. These modifications are real, they work, and they're not expensive for what you get. Tell them Mach V sent you.