Figured out how to engage this game's built-in limiter. You need a copy of Mafia 3: DE to do this.

1) Start Mafia 3 DE, set a 60fps lock (or a 30fps one if you prefer) and save the settings. Exit the game.

2) Replace this file: Documents\My Games\Mafia Definitive Edition\Data\<ID>\profiles\temporaryprofile\profile_videosettings.pf with the one from Mafia 3 (here: %LocalAppData%\2K Games\Mafia III\Data\<ID>\profiles\temporaryprofile\profile_videosettings.pf)

3) Play Mafia: Definitive Edition and change the graphics settings as desired (I use exclusive Fullscreen, VSync off but enabled in Nvidia Control Panel, everything on High)

Note: you can't modify the profile_videosettings.pf file by hand - even with a Hex editor. The game finds it corrupted and overwrites it. A valid Mafia 3 file does work though, hence why this solution is legit.

Enjoy nice smooth 60fps at last :)