Eyeballing the map:

For largest absolute net gain of land area, I guess Mongolia wins the cake, getting a very large slice of Siberia while losing almost no land. For a percentage net gain of land area, maybe one of the European microstates, or East Timor.

Largest absolute net loss of land area is Russia for sure. Largest percent loss is... probably Russia? Again, losing Siberia is a large fraction of its land, and nobody else seems to be so screwed by the distance.

Excluding overseas territories, there's three borders between Yakutia-cum-Japan and its current capital, Moscow, and another case of that in the far western reaches of Brazil. If you include overseas territories, well, French Polynesia is currently almost literally antipodal from Paris, and I don't really know how you would count 'most countries away' in that case, but you can't really get further than that.

BC's intact too, if I'm reading this correctly. We lose some far north to Iceland and the very southern tip of Ontario to the US, and that seems to be it as far as I can tell. And as a trade we get New England, a good chunk of Washington, and the northern Plains and a bit of the Midwest. Not bad, really!