At GCSEs, you should only really be working hard if you're of below-average intelligence. Many find that even if they don't listen in class and don't pay attention, they get As for their tests. These types of people just need to work a tiny little bit to push that to an A*. OP, if you find that you are already the head of the class without doing all this, then doing this will really just push you too far - after all you only need so much to get an A*, an examiner can't give you a grade higher than that. I often like to think I'm one of these people, heh but I'm an arrogant dick XD
If you find that you get mostly Bs and Cs, and aren't at all intellectually special, then working hard is what will get you an A*. To be honest, I consider myself a tiny bit above average, and I manage As in half the subjects I do, with minimal effort. It wasn't until recently that I started actually listening in class - thats when I started getting A*s on a regular basis.
I find lots of people on TSR are of above-average talent, as opposed to many of my peers. Its very hard to judge intelligence, and there's always something arbitrary about it, but you can easily determine if you're bright or not.