I can't prove this but I suspect there will be very little difference.
The wavelength on 80/75 meters is so much longer than the car body that
going from a low mount to a high one will be almost unnoticeable.
Ground loss will be about the same because the capacitance between the
car body and ground is the important factor and does not depend on
where the whip is mounted.

What will matter greatly is the Q of the coil. Make it inherently as
high as you can and keep it away from metal parts of the car body.
Resonate it and match it and you will have lots of fun. 80/75 is a
great band for mobile and much underused.

Bill, W6WRT