And it was rather some other male singer with high profile such as Justin Timberlake or Bruno Mars, could've Stay been even a bigger hit? Or did it benefit from the "unknown artist" effect?
I think initially it would've benefited from having a higher-profile feature because when albums are made available on iTunes etc you're inevitably drawn to those songs out of curiosity and they thus tend to chart higher/sell more than the solo tracks on an album at first.
Generally though, no, I don't think a higher-profile feature would've helped its overall success. It just needed a male voice to provide an alternative perspective to the lyrics and those vocals could've been anyone and it would've had the same effect imo.
It would've been the same without him, or it would've been a bit smaller. That song's an amazing duet because of the contrast of voices, and I feel the general public is eating it up right now in part because of it. It would've been absolutely massive if Bruno Mars was on it.
I definitely think that a high-profile singer would have boosted the song's success no doubt. Think about the hype from the video and related effects (Grammys performance) that would have make a big impact.