Custom tunes improved my mileage by about 0.5mpg over the canned tunes. Canned tunes improved my mileage by maybe 1.5-2.0 over stock. YMMV.
In my opinion, you are not going to see the "bump" between canned and custom that you saw between stock and canned. Custom tunes are more like a "polish" on an improved product.
Hope that helps.
- Jack