I'm a lefty here, too, with my mouse on the left-hand side of the keyboard...personally, I've never had trouble using the quickslot keys where they are. :shrug: I always map the movement keys to the number pad, rather than WASD. (I do this with every game I play that has customizable bindings, actually. It's so much more comfortable for me; otherwise my hands are all cramped in together on one side of the keyboard.) So yeah, it's a small reach to go from movement to the quickslots, but I guess I'm just used to it--my inventory (I) / use (E) / auto-walk (Q) keys are all over on that side of the keyboard anyway. Have you tried taking your hand off the movement keys to hit the quickslot buttons, rather than off the mouse?
(Disclaimer: I also have a smaller-sized keyboard, due to my hands being small and my fingers being on the short side.
So the reach isn't that far for me. If you've got a larger sized keyboard it might be problematic; I'm not sure.)