To answer your questions:
1) Use what the manufacturer recommends...are you smarter than them? I know I'm not. All the spark is doing is igniting the fuel/air mixture...how are you going to get more HP from a different spark?
2) I am sure someone will disagee with me, but I don't see how this is possible. Same amount of fuel is making its way into the combustion chamber regardless of what spark plug you have screwed into the hole. I could maybe see a decrease in fuel efficiency if you were to put in a radically incorrect spark plug that caused the engine to run poorly.
If you are looking for a relatively inexpensive way to gain a few HP by replacing the plugs, I think you are searching in vain.
It is still ugly.