Ethan Slater thinks Ariana Grande is an “incredible” performer. The 32-year-old actor stars alongside Ariana, his real-life girlfriend, and Cynthia Erivo in the new ‘Wicked’ movie, and Ethan has hailed his co-stars as vocal “powerhouses”. He told ‘Extra’: “They’re amazing, they’re powerhouses, they’re like voices of a generation. “Everybody knows what incredible singers they are. You think they’re really different singers and then they sing together and it sounds like one voice, which is like a beautiful metaphor for them in this movie. “They’re also just really connected actors and they’re re…