The BU-303 is DGPs Compliant which gives it an accuracy of 1-5 meters in good conditions. In other words with 4 or more sats. Easily obtainable. So you can count on accuracy of at least 15 feet aprox. If I was you and you noticed that much of a difference why not move it back and just place it on top of your dash, or on the roof. I recently am considering this with my install. It will give you even better accuracy for it will have a better veiw to the sky and more sats. Place similarily to a XM sat antenna. Run the cord along the inside of the weather stripping of your cars windshield and back in through the fire wall. Use the Magnetic mount on the BU-303 for mounting and if you dont like it, no holes drilled so just move it.
ANyway just one of my recent ideas for the same dilema I have.