I just tried it with a no aero C425 Genesis and it made the car much more balanced.
I get an error every time I open the sheet saying "The reference is not valid. References for titles,values or sizes must be a single cell, row or column." It seems to be affecting the aero sheet only, with drag force, downforce and theoretical top speed and overdrive as #Ref! errors.
Edit: I can't access the Google Doc version, I do not have permission. I am going to try the gear and aero tuning on a different car now.