Not sure if this correct but I saw intel's roadmap for kaby lake that includes a 28w quad core. That'll be a great boost for the next 13". Maybe that's why there are two fans in it now? Also, what kind gains are expected for the 28w iGPU anyway?
I don't think a 4 core is in the plans for the 28W chips. Everything I see for the 28W series shows the 2+2 or 2+3e (that's CPU + GPU in a sense, 2+2 is 2 core CPU plus regular graphics, 2+3e is core CPU plus Iris Pro).
As far as performance goes, I think the best thing about Kaby Lake is likely to be the 4K video support, which is supposed to bring power usage while running a 4K video stream down a TON. Otherwise its sounding like a typical incremental increase in overall performance.