Not an insubstantial set of enhancements.
It looks like 2K Games have released a patch to enable PS4 Pro compatibility and enhancements for Mafia 3; apparently, PS4 Pro compatibility was added in the game in patch 1.5, which, as you will remember, added racing and custom rides to the game. For some reason, they decided not to tell anyone about it, though.
That said, Digital Foundry have confirmed that PS4 Pro enhancements were indeed added. According to them, PS4 Pro enhancements include the addition of motion blur and improved shadows, as well as a bump in resolution to 1440p. Notably, none of these changes apply to the cutscenes, which continue to remain the same as on standard PS4 systems.
It’s not as barebones as we have seen some other games’ PS4 Pro enhancements be; owners of PS4 Pros may want to look into picking up a copy of this game, maybe. Mafia 3 is available on Xbox One, PS4, and PC.