I can't say anything about the gamepad, but I own a G600 and it's one of the best purchases I have ever made. I got one after having gone through two Razer Nagas and can definitely say that the build quality is better.

The G600 also has a third main button for your ring finger, which is the best part of the mouse in my opinion. For GW2, I assigned "roll" to that trigger because I used it so frequently. In Planetside 2, I used it to just my DPI (mouse sensitivity) on the fly depending on the weapon I was using. For WoW and SWTOR, I used it as a toggle for mapping another set of 12 buttons on the side of the mouse.

...so yeah, it's a pretty awesome mouse. Is it overkill if you already have a gamepad? Quite possibly. Basically, it boils down to whether you need more buttons or not. If you do, then this will be a good investment. If not, it might still be a good investment, but probably for a game down the road. I don't use all of its features for every game I play.