Think about it in this way:
The cup is always in turbo boost once the system is putting pressure on it. For comparison, a top of line 5557U clocks at 3.4GHz when it is in extreme turbo. So does the 6660U.
As IPC gap between broad well and skylake is not that drastic, a 15w chip performs as same as a 28w chip at the same frequency unless the thermal envelope does not suffice the heat output (in this point of view the 15w chip is even better). Plus its base clock is at 2.4 GHz so so that the system is even cooler and quieter when it is not stressed (therefore longer battery life as well). The base clock of a 5557U is 3.1 GHz, which produces more heat and spin the fan up even more than a 2.7 GHz 5257U.
Considering iris 550 and 540 only differ by clock speed, it is no brainer for a OEMS to choose a 15W over a 28W chip before a 28W GT3e Package can house quad cores.
Just personal opinion

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Not a huge difference this time

Look up 6660U and 6567U at Intel website.
As I have said, as far as Intel only provide dual c in 28W package it is no need to still use a 28W chip.
Besides, since the MBA is going to fade away, the new 13/14 inch should just be a MacBook (if use 6x60 series, competitive to last gen MBP CPU wise and much faster graphic wise).
I could be wrong in the end but I would like to see apple combines the screen of MBP in the form factor of MBA