I'm going to settle in the middle between you two guysNathanOlder wrote:
No way is the Mercedes Engine worth 2 seconds over the Renault. ...
Mercedes Benz
Ferrari +0.8
Renault +1.0

I would go for:
1s vs Ferrai powered
1,2 - 1,5s vs Renault powered based on raw power
Factoring in driveability and consumption,
I would go for roughly 1,5s for both Ferrari and Renault PUs.
The big question mark is harvesting. That could in some instances be repsonsible for the 2s+ observed (at least temporarily) in Bahrain.