Every couple of years companies come out with new design plugs that promise better fuel mileage and more power etc etc. I remember when Splitfire plugs were all the rage. On a dyno with all conditions controlled (basically a lab environment) the plugs may show some gain, but in real world driving conditions you'll never notice any difference. Most people install them and then say they feel the difference or whatever, but in reality its only in their head because they are expecting to feel a difference.
5% mileage increase and 4 horsepower sounds impressive, but if you're getting 20 mpg before the new plugs a 5% increase is only 1 mpg which will disappear the first time you leave a stoplight quickly or put a couple friends in the car. A 4 hp increase on a 350 hp V8 for example is a .01% increase. In an Intrepid with 200hp that would be 2hp. I challenge anyone to test drive two cars back to back and tell me which one has 2hp more than the other. My car runs great on $3 NGK G-power platinums and I'm sure would run just fine on the $1.50 Champion coppers. No need to spend $10 per plug which is what I've seen some sell for.
Now I'm not trying to dump on everyone or be a jerk but I did want to point out how most of these special spark plugs are much more marketing than performance.