Voyager is actually inline with TNG speeds. At Warp 9 traveling 70,000 LY it would take 84 years to get home assuming a non-stop flight. Since they said 70 years ETA then Voyager would be cruising at an average speed above Warp 9 (TNG).
As to Warp 9.9, they tried it in "Threshold" but the structure couldn't take it. As the series continued, it appeared that the top speed of the ship worsened (either due to damage or power issues or both) and she was never able to hit the advertised 9.975 speed. Perhaps a brand-new, pristine and maintained Voyager could do Warp 9.975 but the Voyager probably was too damaged to do this again, IMHO.