The sky is definitly the limit with these engines. While they make a lot of low to midrange power from it'* pushrod design, it definitly lacks on the high end. Even us supercharger owners don't have as much highway power as we'd like. This can be improved, but it all depends on how much money you want to spend.
A good intake alone will make the overall RPM range a bit more perky, and give much more throttle response.
If you really wanna increase your top-end power, you are looking at adding some higher ratio rockers or a cam that takes advantage of the higher revs.
EDIT: Also, please add your year and trim level in your signature.