There's no way of knowing current engine's pecking order. Renault engine didnt make any difference to Caterham form at all, yet Renault engine cannot be ****. Also now it seems Renault engine didnt affect Williams performance at all either (I have to infer Cosworth is actually very competitive engine. Shame they will go out of F1 next year). Meanwhile RBR keep dominating, yet Merc/Ferrari engines cannot be that inferior to Renault engine. All in all it's just impossible to compare performance b/w eingnes, and if you dare compare, the conclusion is that they are equal for the above reason. In F1, in the past 5 years or so, engine itself has been practically standard element.
Also, current engine is hardly irrelevant to 2015 as all manufacutres are established entities with no shortage in funds, and been in F1 for long time.
Maybe only relevant and interesting guessing is how RBR/Newey's propensity to squeeze everything so tight will affect their reliablity and performance in the new era where power train and integration around it will be more important. They are pretty much the only team still plagued by KERS issues regularly, arnt they?