Thanks for the info. I do like the ASRock boards but just read a KitGuru review where the Extreme4 did come in for some criticism............
"Memory frequencies above 2933MHz start to present system instability. While this is not currently an issue for a budget motherboard due to the expense of 2933MHz+ DDR4 kits, it will be an undesirable problem for the future. Poor memory stability also caused problems when trying to push the CPU and CPU Cache frequencies during overclocking.
Voltage accuracy also presents a significant problem for the Z170 Extreme4. When manually adjusting the CPU VCore, or even when applying on of the four CPU overclocking presets, a core voltage overshoot in the region of 70mV was registered. Seeing your CPU being delivered 70mV more than you asked for is unreasonable. It takes the operating voltage from a sensible 1.35-1.40V up to a more worrying 1.42-1.47V."