I saw the Merc overlaid on the Ferrari a day or two ago, can't remember where now, showing how much more compact the rear bodywork is. In fact the Merc has a little bulge each side which is only there to meet some daft regulation.
Plus, thinking about how Lewis put an easy 0.5s into Nico in Sepang and then Nico used his 'Dummies' Guide to Beating Hamilton' ( ) to catch up, I do think some of that is the driving, and how the drivers combine.
I think it gets into a kind of feed-forward loop, where the benefits multiply. The more developed car does more testing miles, there's more driver input, more time to optimise everything, do your software for performance rather than reliability etc etc.
It's still hard to believe how big the gap is though. And did Merc really keep the split turbo concept secret for so long?