Faster per watt I agree. Per clock, they could be somewhat similar with caveats. Per Core, the clock speeds that can be achieved with intel are much higher (even if it throttles). I think both are good enough for 90% of the population.

I don't see anyone building an external compute/accelerator platform around an ARM Mac ecosystem. It's a niche market as is and the direction has always been to integrate functions like this onto the chip / package directly. I think the latter is certainly an opportunity apple will choose as they will be able to claim their new processors have a special sauce that intel does not have and charge an apple premium for it.

Are you suggesting plugging in a dongle that has a CPU on it? I'm very confused if true. As the bandwidth between RAM and CPU is ~50GB/s, any external dongle would not be able to communicate with the system RAM without a huge slowdown, so the dongle would need it's own RAM too, how this would communicate with a GPU is questionable again and this is already a weird crazy expensive dongle.

I think this is the part that will make 3rd party developers give on on the Mac. Mac ports are given the least love as is, mostly because apple will break your application on such a regular basis it's not worth the cost to port/support, but this change adds so much more burden to the developer, I think most will wait a few years before making a commitment unless apple throws money as them.