I think the map is perfectly set up right now with the stalemate line it has for the rules that currently apply. If it was 17, winning would be dramatically easier. If it was 17 the player soloing would have to worry dramatically less about crucial solo aspects, such as:
-When to stab England and how to limit his power while maintaining alliance (When playing as Germany)
-When to stab the juggernaut you're participating with.
-How soon to sneak into Tunisia, as a European country

Most players get stuck at 16 or 17, so I think 18 is the perfect number.

My thought is that the first player to reach 17 SCs would win a solo victory. If two players simultaneously got to 17 SCs, they would automatically 2WD.

Would the game be better or worse with this change? Would it be more or less balanced, and what would the ramifications be on how the game was played?

I am unsure. On one hand, I think it's very interesting that a large part of the game is sneaking into a draw as a minor or waning power. On the other, solos are fun and this might even encourage more interaction across the stalemate line to interfere with a growing player, rather than a race to seal the stalemate line once someone starts growing out of control.