A hybrid should not be faster downhill because it is heavier.
At first glance that does seem to make sense, but it doesn't.
What is the difference in weight? a few pounds.
That should not make it considerably faster downhill taking the reduced aerodynamics into acount.
If you go about 40 mph downhill your biggest problem is aerodynamics and so a roadbike should always be faster especially at those speeds regardless of weight.