Gotta remember that all these previous ARM chips have been designed in the context of the phone. If you suddenly tell the chip designers they have 10x the power and thermal headroom, theres a lot you could do.
Apple's A CPUs are already the best in their category, by far.
Knowing Apple had Intel Macs running in the background for years before they announced the jump to x86, I wouldn't be surprised if they already have the chips and know how well it performs.
