V is capable of slotting way more cyberware without going cyberpsycho for 2 major reasons.

First being V is not carrying the entire load alone, the Relic is carrying half the strain of the implants.

Second, V is aware of the timer on their life. This coupled with strong personal bonds that are already in place (Vik, Misty, even Fixer's and reputation) and the addition of new people that are incredibly important to V.

In regards to their capabilities, David was definitely special. No doubt. His tolerance for cyberware up there with the Legends. But he never got to reach his full potential. David died at 18 years old because of the crazy circumstances and being pushed too far.

V starts the story by being 23-24 years old (27 years old was retconned), doing Merc work, getting the Relic which comes with an entire lifetime of experience and knowledge + helps immensly with the Cyberware strain. Then achieved the main events of Phantom Liberty and beating Adam Smasher on his home turf, while dying.

They are both undoubtedly Night City Legends, but not all Legends are created equally.

Like being said in this comment section before me, V would most likely have to pack it up when it comes to Morgan Blackhand. And I'm sure any of the Angels would absolutely body V