[speedy]

putting more weight in the front does increase rear bias by default, this is a good solution to improve beetle braking by shifting a little weight over the front wheels improves grip therefore increasing the braking torque that can be applied ,many people place batterys in the front to accomplish this without adding weight to the car .
A bias valve will only work if the car is rear biased to start with !
Like you said each car is individual , wether you have a spoiler on the front or back, oilcooler up front , tyre size ratio and compound full tank of fuel etc
I think it is down to trial and error to get the balance you are happy with .