That's an easy one to answer: BECAUSE IT'S FUN. The same reason people have modified their cars from the beginning. The same reason people modified all the old school cars from the 50s, 60s, and 70s. Define "high powered car." Compared to most of those old school cars, this 2.5L Mazda3 is pretty damn quick. It's chassis is also far superior and far more rigid. This reminds me of a comment made by one of the guys at Kugel Komponents, the company that makes the V8 conversion kit for the first-gen Focus. Someone (a Mustang fan) asked on their forums, "But can the Focus handle the torque made by the Mustang V8?" Their response was great, and something that every old school muscle car meat-head should read (paraphrased from memory): "Are you kidding? The Focus chassis is far and away stronger than the Mustang's!".
I don't understand people who insist on throwing around the (somewhat racist in origin) term, "rice" when people modify these kinds of cars. They're better, faster, and safer than most of those cars that so many baby boomers remember so fondly. Most of those cars were mundane, crude (even for their time) boring transportation machines with big engines thrown into them (which were often not that fast after catalytic converters came along). At least with a car like the Mazda3, you're starting with a car that has an exceptional chassis underneath it to begin with, recognized as one of the best-to-drive FWD cars around. I have a harder time understanding modifying cars like Toyota Corollas.