They don't need to be 2x faster than any Intel chips, just the ones in products they're replacing. And as it so happens it's pretty easy to get 2x the performance over an essentially passively cooled and severely power limited Intel chip that's in the air.
Absolutely. And I don't think it will be difficult at all for Apple to crush Intel in benchmark performance per watt—let alone real world performance with their more targeted silicon.
The interesting question is going to be how Apple's GPU stacks up to the competition. We know from the A13X that it is already quite good and I fully expect it to exceed Intel embedded graphics. But what about higher end machines? How long till Apple GPUs can take the fight to discrete graphics?
Why Apple cannot use discrete graphics? We don't know that yet. NVIDIA already starts to have support for aarch64 so you can expect AMD not that far away.
The video explicitly states that Apple Silicon based Macs will use a shared memory approach between CPU/GPU which is a strong indicator they will go all-in on a combined chip or at least on-die approach.
My bet is that for the iMac & MacPro, Apple will use custom AMD on-die GPUs (like the now discontinued Kaby-Lake G). They just haven't announced it yet and AMD seems to be silent about it. I also bet the demo they showcased of Rise of the Tombraider was running on such an unannounced custom chip.
I you think about it, it doesn't make much sense for Apple to enter the highend segment for GPUs and sink a significant amount of research cost into it. What they need is power-efficient GPUs for their mobile line (iPhone, iPad, Macbook Air & low end Macbook Pro) so that is where an all-Apple GPU design will be used. But for professional high end use, they will not be in the position to ship a GPU (no matter if on-die or discrete) that could compete with AMD or Nvidias performance hogs out of nowhere.
Market share for high-end Macbooks + iMac + MacPro is just too little given their overall product portfolio (including mobile devices) and the research effort too high. Also, AMD would probably happily design them a custom GPU chip for on-die usage, just as they do for consoles and did for the Kaby Lake G. It wouldn't hurt AMDs business, even if Apple plans to replace AMsD with custom all-apple based GPUs in the long run (i doubt it), as it is very unlikely Apple will ever license it to other CPU vendors or even enter the market for discrete GPUs for Wintel machines.
