Gin was able to get the drop on Zeff only because of the latters wooden leg. If he had two healthy legs, Gin would not have been much of a threat to him (yes, I can agree that it's a little assumptive but I stand by it). Zeff didn't lose his strength when he lost his crew, he just lost his reason to sail the seas. Because he felt that without his comrades, it would have been for naught.
I personally believe he did lose strength, or at least the drive to keep growing in strength, when his crew died. To me it appears that he let his skills rust and he didn't continue to develop them despite his handicap, unlike Shanks who didn't let his loss of an arm deter his dreams of sailing the seas(it helps that his crew didn't die as well). Instead, he spent his time focusing on building and running the Baratie and only passing his technique to Sanji.
Had things turned out differently and Zeff loss his leg, but managed to keep his crew, I could see him simply finding a stronger material to replace his lost leg with so that his kicking strength wouldn't be halved.
Kuzan is able to handle his handicap so well because of his ingenuity and Devil fruit. He's made an ice prosthesis that works just like a regular leg would.
Zeff did as well though(granted one of poor quality), but his functions as a regular leg too. MJ3 mentioned that anyone who loses a leg, losses a ton of power, and I mentioned Aokiji as someone who lost a leg and kept his power. You can throw Shiki in that category as well.
Moriah however is a whole other story. He was so psychologically damaged by the utter defeat and slaughter of his crew by the hands of Kaido. He started using his Devil fruit to make zombies because he was so afraid of losing his comrades all over again, he wanted an army of zombies that didn't feel pain.
I used him as an example because it shows that a pirate losing his crew does have an effect on his psyche and that can extend to his strength. We saw the same thing happen to Luffy, although he was able to realize that his crew was alive and well, unlike Moriah, until he lost Ace, where, again, he realized he still had things to live for(his crew). Moriah switched his goal up and stopped focusing on his own power, instead relying on others.
Shanks still has things to do and reasons to live for. His crew and Luffy are alive and well and he is aiming to be the Pirate King(possibly). Zeff lost his crew, his ship, and his dream to find the All Blue himself. Shanks continued to grow. Zeff didn't.
Kuzan can also create feet from ice... Shanks is powered by ambition and his hope in Luffy, but is that enough to reach the potential he had with two hands?
I don't see why not. Just because one path is closed, doesn't mean all paths are closed. Would he have to work harder? Most definitely, but it can be done. Especially with his ambition being as high as it is.
Long story, short...losing a limb in One Piece doesn't appear to mean squat when it comes to certain folk, so Shanks can still potentially go toe to toe with Mihawk when it comes to sword technique.
Even though it didn't handicap him strength-wise, its possible that Mihawk still would see it as an insult to his honor to fight a one-armed man. Possibly Shanks just accepts that, too.
