[aerohead]

This is awkward for DIY's.
An automaker knows the road load and BSFC for their vehicle.If they plan to modify it,they can predict it's performance numerically.
If you had intentions to fully trick out the Focus aerodynamically at some point,there would be a practical issue of going ahead with all that,and then modify your gearing to control the BSFC.Altering the engine isn't necessarily required.Just gear-matching. So says the automakers.