Not really an infinite tank. He can take more damage for sure, especially from blunt objects or hits.
But think about some of his future potential opponents:
- If he fights Kizaru, he's primarily a physical fighter that moves a lightspeed and creates lasers. Luffy still hasnt mastered FS yet, so that could be a problem.
- If he fights Akainu, G5 isn't directly tanking his devil fruit if he gets hit without CoC coating.
- If he fights Shanks, he has a sword. Different from Kaido's Kanabo. He still gets cut as evidenced by 1047 where Kaido launches wind scythes at him.
- If he fights Blackbeard, his powers can be fully negated with Yami Yami
And who knows what Im could have if anything. Id imagine something horribly broken if they are a fighter.