It's important to know that overlining or underlining a fly rod will not change it’s intrinsic properties. It will, all things being equal, make it bend more or less for a given amount of line out the tip top. Here's the lowdown...
Bill Gammels 5 Essentials of Good Fly Casting. I have always maintained that if you want to improve your fly fishing the place to focus your effort is on improving your fly casting and presentation skills