This is my opinion, and I'm not a knife or sword maker, but I suspect that you wouldn't see any performance benefit from differential hardening in a "knife size" blade. Sure, you can strip and patina a differentially-hardened blade for a cool hamon, but it takes a special sort of person of stress one of Dan's knives enough to snap it -- I'm not talking a chip from hitting a rock, that'll happen regardless of how the blade is hardened, I'm talking about literally breaking the blade.
Unless I'm very much mistaken, enhanced blade flexibility is the only performance gain from differential-hardening; I've read that's why Swamp Rat went to through-hardening after the first Battle Rat run -- there just wasn't enough benefit to justify it. If you want a traditionally made "live" sword or or axe, then you'll want it differentially-hardened, but modern blade steel quality can be good enough for even high-quality swords to be through-harden. I also suspect that the spine of a through-hardened knife is better as a fire steel striker as well, so a case can be made from both a cost and performance perspective that through-hardening is better than differential for knives.
