Challenging, but perhaps not in an interesting way. The only significant change is the addition of Agony and some new attacks; otherwise, it's just more health and damage.
Recently played Witcher 3 with Item cheats. The greatest video game ever made. Hearts of Stone and other books fared well. There is the use of Witcher 3 console commands and Witcher 3 item spawn codes to make obtaining some high-level equipment easier. Geralt's crossbow damage may increase.