Congratulations. Very cool car! Difficult question to answer. A few years ago my son and I did some re-prep in hopes he could pick up some speed at districts ... sadly we actually slowed the car down. I ran this years pinewood derby for our pack and a few parents suffered the same experience when trying to improve performance. On the flip side, last year during test and tune the night before the race we reviewed in slow motion the video of my scouts car and noticed the front wheel was riding away from the rail in a few spots on the track. I rolled the dice and added more steer ... luckily the change paid off. With all that said I'm not a professional racer and I would take any advice they offer over mine.
Best of luck to you and your scout.