Maybe consider a new chainring (or rings) up front? If you need more speed then a larger 'big' ring might help. IF you need to go lower for climbing hills then a smaller 'small' chainring would be the answer. Just adjust the FD either up or down as needed. And I'm thinking it would be cheaper and quicker than redoing the whole rear set-up and shifters. Just my 2 cents worth for your consideration.