I think they can very likely match the TDP-down (7.5W,800MHz) performance of a Intel CPU from 2015 in a passively cooled iPhone. Now can they match the TDP-up (25W,2.8GHz,actively cooled) performance of this chip? Probably not.

That's the thing with Intel CPUs. There are a huge number of ways to configure them. Even knowing the part number is not really enough to judge what the overall system performance will be.

https://en.wikichip.org/wiki/intel/core_i7/i7-6600u