Yeah it will be a long time before you can try to level grind off them. That's the main reason why Zaheer felt so OP till he fought Tenzin, I would imagine Lin could probably take him, but that's probably about it.
I couldn't see Lin or Su defeating Tonraq though, and he couldn't hang with Zaheer (sure it was 2v1 with a handicapped Korra, but whenever Tonraq was in the ropes 1v1 versus Zaheer Korra ran interference).
I actually loved that fight for how well Korra and her father worked as a team.
Still, different bending styles means a different fight so who knows.