Best temperature to freeze pie filling?
Whether colder is better would depend on how freezing "simplifies production" for you and how long the food will be stored.

In a long-storage scenario, lower temperature is definitely beneficial as it does improve storage lifetimes. If it's a matter of weeks or days so that you can prep a batch and cook as needed, no real benefit.

Without knowing how freezing "simplifies production" for you, it's hard to say if colder would be of any benefit on that mark. All other things being equal (as they often are not) a colder freezer will take more energy to run, since additional refrigeration is not free. 

My sorbets typically hold more than long enough at 0°F. If the mixture is not churned, it freezes hard at that temperature. Some sugar syrup will weep out, but that will also happen at cooler temperatures. Even if churned, it would not slop out of a pie shell, so I'd say it would handle similarly.

{not that you seem to be considering one - but mention might be made of "blast freezers" which are sort of the convection-oven of freezing, and may run as cold as -70F - those have the benefit of very fast freezing, minimizing the size of ice crystals - then the product is typically moved to a storage freezer once it's been "flash frozen" - not clear that this would offer your product any benefit, as the filling will be cooked after thawing. }