Out of curiosity, how would they produce more power? I assume the regulations regarding maximum rev limit/fuel flow per unit time haven't been changed since last season. I'll admit I have limited knowledge of these new hybrid/turbocharged engines, and I'm definitely glazing over the hundreds of things that could be tweaked to create more power, but my understanding is that rev limit + fuel flow are two of the most effective ways to limit power production and the regs still have them locked down.
Also, in an interview Toto Wolff said their cars did over 6k km in testing on a single engine ("power unit"). I don't how much of that is PR semantics, but I can see what you mean about increased confidence in their engines (at least in Mercedes' case--other teams didn't fair nearly as well in testing). The commentary team also mentioned that Hamilton's pole-setting time was only ~2s off Schumacher's all-time fastest lap at Albert Park (back in the V10 era).