Hello! There is no "user friendly" method to adjusting when the Active Fuel Management switches between 4 and 8 cylinders. However, there are softwares out there, that with a laptop, will allow you to "modify" how the Powertrain Control Module operates, and from there, you can adjust many of the settings, including the requirements for switching to Economy Mode. One such software is HPTuners (www.hptuners.com) You have to purchase the software and the cable and have a laptop, but once setup, you can make many modifications.
Increasing horsepower doesn't typically increase economy. In fact, items like superchargers will increase fuel consumption. However, some items that increase horsepower, do so by making the engine more efficient, which in turn help with fuel economy. For example: ehxuast systems. Manufacturers are requried to meet certain noise levels, and therefore often have restrictive exhaust. An aftermarket exhaust system will help the engine breathe better, which frees up some horsepower and the engine runs more efficient.
You can also buy hand-help programmers that are specifically designed for improving fuel economy (hypertech and superchips are two common ones). These are basically like the HPTuners I mentioned before, only they are "pre-programmed". You can't make the changes you want, only whats in the programmer. Its less expensive and easier to do, but not as flexible.
I hope this helps, please let me know if you have additional questions!