New York State (I don't think a city-state Hong Kong deal would survive) would essentially be OTL Canada.
Many of the loyalists who founded what we know of as Canada would probably go to NY instead. As would many of the loyalists who went to Europe and the Caribbean, I imagine.
ETA: I think "West Jersey," Manhattan, Long Island, Staten Island and the Hudson Valley up to and including West Point would be the absolute minimum.
Do you really think that the New Englanders and the people of the Middle States are going to permit the bisection of the colonies and the isolation of New England from the other colonies?
It is true OTL that many of the Loyalist New Englanders and New Yorkers settled the Ottawa Valley, the area around OTL Toronto, and the Eastern Townships in Quebec. I would expect a similar exodus, even if the newborn United States keeps most of NY State.