I think GeekBench is a poor benchmark, since it's measuring peak CPU performance in a narrow domain, but the table you cited shows that the 4770 is 6.5x faster

I think GeekBench is a poor benchmark, since it's measuring peak CPU performance in a narrow domain, but the table you cited shows that the 4770 is 6.5x faster than the A7 (16774 vs 2564). Even at the same frequency and core count, Haswell is 1.8x faster in the benchmark. Whatever the exact number, it doesn't invalidate the point I was making: Switching to ARM will ruin performance for x86 programs, which need to be emulated. ARM has to be significantly faster than x86, or else the emulation overhead will make users hate the transition. This is a huge challenge for the Mac Pro and rMBP, which are all about performance. Generally, a given microarchitecture can only scale TDP by a factor of 10 by changing frequency and number of cores. Satisfying the pro line would require a new microarchitecture. Considering an x86 to ARM transition would take 1-2 years, that's asking a lot of Apple's chip designers. If Apple was going to go through all the trouble of building a new ARM core for high-performance, high-TDP applications, they might as well build their own x86 CPU. It would save a transition and the necessary emulation overhead. They could roll it out for the MacBook Airs first, then address the Pro line at their leisure (or not and just use Intel CPUs).