I voted no. Definitely first-tier, but not OP. Basically, like others are saying, they can do anything pretty well, as opposed to one thing really really well. That's why the AI tends to be a runaway civ when it's Poland: the AI can't prioritize single VCs very well, let alone beeline specific policies and techs. As a result, a strong, blanket boost helps it a lot. On the other hand, if you're going for a specific VC from turn one, there's another civ that can probably do it better.
Domination? Zulus or Huns
Science? Korea or Babylon
Cultural? Brazil or France
Diplo? ALEX ALEX ALEX.
So Poland's tier one, solid overall, but not overpowered.