The A15 efficiency cores will be in the next model. They are A76-level performance (flagship-level for Android from 2019-2020), but use only a tiny bit more power than the current efficiency cores.
At that point, their E-cores will have something like 80% the performance of a Zen 1 core. Zen 1 might not be the new hotness, but lots of people are perfectly fine with their Threadripper 1950X which Apple could almost match with 16 E-cores and only around 8 watts of peak power.
I suspect we'll see Apple joining ARM in three-tiered CPUs shortly. Adding a couple in-order cores just for tiny system processes that wake periodically, but don't actually do much just makes a ton of sense.
