You do not need to add an additional factor above whatever the calculated latent and sensible heat loads are. Adding more refrigerating capacity by itself won't really do anything (like IRStuff said, your DeltaTime might be unreasonable though making the simple load calculation worthless). How a carcass freezes depends mainly on the room temperature and air movement in the room. There are some examples in chapter 20 of the ASHRAE Refrigeration handbook I think; it boils down to a heat transfer problem.
There are two main ways to decrease the time it takes to freeze a carcass. Increase the temperature difference between the cooling medium and the carcass: make the room colder. Increase the convection heat transfer between the carcass and the air: use high velocity fans and circulate the cold air efficiently. There are some blast freezing examples somewhere in the ASHRAE handbooks and there is some guidance on air velocities... but I think you have to be prepared to do some heat transfer calculations in order to make sure the DeltaTime you "pick" is actually reasonable. So... in a round about way adding capacity in the form of making the room colder will help, but simply using bigger refrigeration equipment and keeping the room temp the same will only result in the unit cycling more and won't help your heat transfer at all. Trick is figuring out DeltaTime.
