Well, "cutting" Katko's seat doesn't necessarily mean eliminating a Syracuse-based seat or otherwise chopping up Onondaga County. A Syracuse-Ithaca seat is pretty natural and is unwinnable even for Katko.
Katko won by 13,700 votes in 2018. In 2016, Tompkins County gave Hillary Clinton a margin of 16,000 votes over Trump, with Stein and Johnson taking 3% each. I suppose if Dems take out strong Republican territory to put it in Katko's district, I can see it. The Southern Tier district would have to take in Binghamton from NY-22 and probably even more than that.
It's not just adding Tompkins County and taking nothing away. Wayne County gets dropped from Katko's district, so it's a net swing of 26,000 votes from Trump to Clinton. Katko can't overcome that. As Mr. Phips noted, Katko lost Onondaga County itself in 2018.
It actually results in a split of Binghamton, which isn't great but doesn't scream gerrymander either. You could probably fix that by splitting a different county in a more natural way, like splitting Cayuga County and then taking in Oswego, since the Erie coastline of Cayuga County isn't really connected to Auburn in any meaningful way. I've made some maps to this effect before. But exactly what would have to happen is heavily dependent on the Census figures since the estimates are hard to be sure about. Maybe you could get away without any meaningful splits.
