Only setting so far that needed more was with one stick of F4-3200C14D-16GVR that needed 1.56V for 3733 13-13-13.
I can do the math on MHz compared to cas latency to determine what the fastest combo of frequency and CL is, but if I tried with the timings left stock for 4000mhz 15-16-16-36, what would the voltages typically need to be increased to, to run at 4200 and 4400mhz?
My F4-4000C15D-16GVK 4000 15-16-16 1.50V does:
4000 15-16-16-36 1.47V
4000 15-15-15-32 1.50V
4000 14-15-15-32 1.50V
Increasing frequency at 15-16-16 from 4000 to 4200, tCL 15 will be your bottleneck. It might very well be that 4000 is your max. But with good ICs you might get 4200 at 1.55V. With not very good ICs your best bet would be:
4266 16-16-16 1.52V
4400 16-17-17 1.55V
Why G.Skill is doing XMPs with higher tRCD and tRP is probably because B-Die doesn't scale as well with tRCD/tRP as with tCL. At least at 14, at 15 tCL is harder to run and at 16 tCL and tRCD/tRP are around the same.
For example my best kit, F4-3600C14D-16GTZNB does:
4200 14-15-15-32 1.55V, that is a whole 300 MHz/tCL, but only 280 MHz/tRCD or tRP.
It also does 3900 14-14-14 1.48V which is again 279 MHz/tRCD.
Though this doesn't show at G.Skill XMPs, only when you overclock. As best G.Skill MHz/tCL is 4400 16-19-19-39 at 275 and best MHz/tRCD is 4000 15-16-16-36 at 250. 250 is not really good, even my worst kit does 270.
All of what I said is for 2 sticks. With 4 sticks you should be happy it works XMP, maybe try lowering latencies like I said to 14-15-15.
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