The 4 core i7''s are about 20% faster clock for clock than an i5, but only with all of its threads utilised.
The i7 are the same as the i5's, they just have an extra thread to each core adding about 5% to each core
In DX11 DrawCalls only one thread is used, in the case of an i5 its 25% of total power, for an i7 its 20%, for an FX-8350 its 12.5%.
DX12 will use 6 or more threads, for an i5 its anything up to a 400% increase, its a bit more for the i7's with all 4 main threads and 4 hyper-threads, about 500%, for an FX-8350 its an 600% increase.
(And thats not counting lower overheads)
Of course that still put it behind an i7, but the performance is a lot closer and with that may DrawCalls its going to be impossible for the first few years to saturate any of those CPU's.