Now, if you are looking for massive power gains with EFI, you'll not be real happy with the results. A properly tuned carburetor will pretty much match what an EFI system will produce in terms of peak power output. But because you'll have very precise control over fuel and timing, you will be able to push the limits closer to the edge and maybe run a little more compression, a little more timing, and maybe a little leaner than otherwise would be safe. You will see improvements in part throttle operation and probably also in the lower end of the power band because, again, you will be able to control the fuel and spark much more exactly than you could ever even think of with a carb and a distributor.
Another benefit is the ability to record and switch between more than one tune. You can optimize one set of maps for general cruising on pump gas, and another set for when you are feeling frisky and want to dump in some C16 and go racing. You should also be more consistent since the XFI system constantly adjusts the fuel for differing temperature and weather conditions. No more needing to change jets because it's cooling down at the end of the day. And if you run the data logger, you can go back and see exactly what your engine was doing during the entire run. The XFI system is also very adept at handling blowers, turbos, and/or nitrous applications. Just give the guys at FAST a call, tell them about your dream engine, and they'll set you up.
In the Air/Fuel Ratio table, you have to tell the ECU what A/F ratio you want your engine to hold at every rpm and load that it could ever see. The program will compare these numbers to the actual A/F ratio as measured by the wide-band O2 sensor and will add or subtract fuel to make the actual A/F ratio match this table, at least within the correction limits that you have set.
What's critical here is that you know what these numbers need to be for your engine. You'll likely tweak them some during your dyno session as you are looking for max power, crisp part-throttle cruising, and a clean idle, but you must start with some safe ratios that are at least close. In a lot of scenarios, the folks at FAST can set you up with good starting base tables that your dyno operator can massage to his liking. There are also independent tuners out there that will provide you with tables based on their experience with similar engines. In the end, though, accurately reading your plugs is still the best way to know what your engine needs.