Kaby Lake with GT2 would be a downgrade from Skylake with GT3e/GT4e . There will be a few percent computing gains at most, and the loss of a significant part of graphics performance that GT3/GT4e provides - upto 2x greater performance with GT3e (which has 64MB eDRAM that GT2 does not, in addition to 48 EUs vs 24 in GT2), and even greater performance from GT4e, which has 96 EUs and 128MB eDRAM.
The average user would notice the lost graphics capability with Kaby Lake GT2 much more than they'd a 2-3% gain in computing rate that most people would never 'notice' if they weren't told about it beforehand.