I find it difficult to put an exact number on it, but he's certainly enhanced in strength to a noticeable degree. I think it's important to show how he's presented in-universe more than anything else when establishing the opinions of the writers/developers.
Here are three sources, one from CD Projekt RED's website and the other two from the Blood & Wine DLC that state witchers have their physical strength boosted as part of their mutations, and in the DLC Geralt has his mutations boosted further so it's safe to say he's clearly above peak human.
As shown here. At 0:41 Adam gives his backstory and explains why keeping strong is so important to him. He's much taller and larger than Geralt and is - with the exception of somebody who cheats - the best arm wrestler encountered in the game and a peak human in strength. Though skill plays a large part in arm wrestling in real life, comments on Geralt's strength are made throughout the video and after Geralt wins the match at 6:19, Adam comments specifically on how strong Geralt is and how if the Nilfgaardian he fought was as strong, he wouldn't be there to arm wrestle. Further confirmed in the quest log for arm wrestling:
What a fight it was! Adam Pangratt's technique was excellent, as was his grit. He only lacked mutagens coursing through his veins.
Adam's lack of enhancements are attributed to his loss in arm wrestling.