AP is an abbreviation for Attack Potency, which is listed on the page of characters.
Amp is an abbreviation for amplification. A speed boost is often called a speed amp, and a damage boost an AP amp.
Right now, Shadow's attack potency is more than infinitely greater than Kirbys. Once that gets fixed, they should be about the same, as they are both Multi Solar System level and I don't remember either having a calc.
However, as you'll notice, speed is equalized. As such, neither will be faster than the other.
As for the star rod, it being "just AP" is because of how the site works. You'll learn quick that a diverse selection of abilities, referred to as "Hax", is generally more important than raw destructive power.
Hope that's helpful, i'd suggest reading some of the pages listed on the front page under the newcomers tab.
If you're wondering about stuff, feel free to ask a staff member. We're the ones without blue names, and a complete list can be found on the page fof
VS Battles Staff. I'd avoid disturbing the ones with purple or yellowish-orange (DarkLK and Don'tTalkDT) names for minor things though, they've got a lot more to do in their roles.