I couldn’t locate specific impact energy data below room temperature in the literature I have for 17-4PH in this heat treatment condition. At room temperature, the value reported is about 20 ft-lbs minimum CVN impact energy at the 900 heat treatment condition*. Below room temperature, the impact energy for this heat treatment condition is going to decrease in value so if there is a so-called DBTT it might be above RT, if you can find CVN data at elevated temperature.
*Source; "Handbook of Stainless Steels" by Peckner and Bernstein