Larian Studios Forums Divinity: Original Sin 2 Divinity - Original Sin 2 - General Steam Pre-Order

Is there any reward for people who bought the game before release? I'm talking about steam, not kickstarter. Maybe access to full released game one or two day early?

Yes - the fact you got to play Early Access this whole time.

Nothing beyond EA access. I do believe if you purchase before release there will be a special dlc that you can purchase to upgrade into the digital collectors edition. But the CE doesn't give any in game benefits, just the ost and a few other goodies.