If Apple could persuade Intel to play foundry for Apple ARM designs aimed at the 15-45W power envelope (which, at this point, are not publicly known to exist), it's not hard to imagine that Apple would seriously consider the option. But Intel taking that deal strikes me as highly unlikely.
So if Intel won't do it, who would (or could)? That would be the other foundries with leading-edge High power/performance manufacturing processes, of which there are three(-ish): TSMC, GloFo, and Samsung. The problem for Apple is that those foundries remain at least one process generation behind Intel for higher power envelopes.
The GeekBench benchmarks that you're seeing, where an iPhone X scores slightly under a MacBook Pro on single core (by core I assume they mean threaded) performance and slightly over on multicore performance, compares the A11 against an i5 7267U. That's a a dual-core, SMT chip, listed at 28W TDP, manufactured on Intel's 14nm+ process. In those systems (the system, including screen resolution, seems to have an unusually large impact on GeekBench scores), the A11 and i5 7267U score, respectively, 4197 and 4223 on single thread, and 9861 and 8947 on multithread, performance, according to Geekbench. For comparison, the i7 7700HQ (4 cores, 8 threads, 45W TDP, 14nm+) found in the
15in MacBook Pro scores 4356 and 14400. A stock Ryzen 7 1700 (8 cores, 16 threads, 65W TDP, 14nm GloFo) scores
3890 and 19935.
Apple's A11 is manufactured on TSMC's newly launched "10nm" process (which is slightly smaller than Intel's 14nm process in terms of feature size). One reason that Apple is able to ship product on TSMC's 10nm process is due to the A11's low power and relatively conservative (in terms of die size) design. Apple's recent A-series chips devote more die space, compared to high performance Intel parts, to cache memory, which is significantly more dense than logic. TSMC's 10nm process is not yet available for higher power (and lower density) designs, and the comparatively memory-dense A-series SOC designs mean smaller die sizes and higher yields. This is worth bearing in mind, given that we're talking about scaling Apple's designs up for increased performance and higher power envelopes (to feed additional power hungry logic, of the sort that would be competitive with something like the i7-7700HQ or comparable chips in the forthcoming 8000-series).
With that in mind, an interesting comparison point will be available next week, with the release of Intel's Coffee Lake architecture on its 14nm++ process. That process will actually have lower feature density than the 14nm+ process, but is expected to have significantly better performance/power characteristics (reportedly comparable to what Intel expects out of its 10nm process at launch). That is, it will give a sense of where Intel is currently at, from a performance/power standpoint, on a process optimized for higher power designs.
To circle back, the question is this: it's one thing to create a great design, bespoke for your needs, with all the hand layout and optimizations that requires, for a chip that targets a low power / high density manufacturing process. Though that's not to say that Apple couldn't create a higher power, more logic-heavy bespoke design that, on exactly the same process, that wouldn't perform better for Apple's needs than whatever Intel has designed for the same target market. In fact, at this point, I'd be surprised if Apple couldn't.
But even in the aggregate, architecture, design, and layout improvements over a prior design have a hard time matching the performance and power gains that would come from simply moving the old design, with no changes, to a new process. As a result, if Apple can't get someone to manufacture its latest and greatest designs, on the schedule it wants, on a process comparable to what Intel is using, it's hard to see the ultimate benefit (that couldn't otherwise be had for a much smaller investment in custom off-CPU hardware) of trying to use design resources to compete with or match Intel's process technology, even if that means using an Intel design and architecture that are inferior to what Apple could come up with starting from scratch.