I'm moving from 7700K/Z270I to 8700K/Z370I and noticed a few quirks:
1. If you set AVX offset to some value (e.g. -3), when you set it back to 0 the value doesn't actually change. You can verify this by stress testing with any AVX program and watching the multipliers drop. The only way to actually go back to 0 AVX offset is "Load Optimized Defaults" -> "Save and Exit" then go back into the bios and load your settings again.
2. The RTL and IO-L values are weird for identical memory timings. I went from 60/61/7/7 on Z270I to 68/68/14/14 on Z370I. As a result, the latency went from about 36-38 ns to 41-43 ns. VCCIO/VCCSA are tuned correctly and both systems are stable (stressapptest -W -s 3600). Here's a screenshot comparison (big thanks to @Jpmboy
for sharing his Kaby Lake memory timings a while back):
Motherboard: STRIX Z370-I
UEFI Version: 0426
Memory: F4-4266C19D-16GTZKW, 2x8GB, @4133