The Z-Move will not fail. This is because all offensive Z-Moves (Savage Spin-Out, Devastating Drake, etc) are actually individual moves that are activated by Z-Crystals. These moves always have:
- No effect besides dealing damage
- A certain base Base Power depending on the Base Power of the original move
- 100% Accuracy
- 1 PP
Therefore, although First Impression only works on the first turn, its Z-Move will work during any turn because Savage Spin-Out never has any special effects.
If you're still not convinced, here's proof.