The heavier fly will take more energy to increase its RPM. So it will take longer to get up to the same speed as a lightweight fly (i.e. slower up hill). So a car with a lightweight flywheel will still get uphill quicker than an identical car with a stock flywheel, in both correct gear and wrong gear situations.
But once you let go of the accelerator, the heavier fly will take you further because it reduces the effects of engine braking.

