I've been pushing it further, if you thought that was amazing ...
This is 24/7 stable - 4200 MHz CL16, also currently with 600 tRFC. It takes 1.53125v DRAM and 1.15v VCCIO.
This might be a world record for 2x16 Gb at CL16?
4266 wont get stable at any looser timings or 1.55v, maybe due to 16 Gb sticks or IMC limit.
I paid just £150 for 2x16 Gb 3200 Mhz LT Sport, but this was before Crucial started binning any higher and I had no clue when buying it what Micron E Die was, I just wanted something better than my previous 2666 kit that maxed out at 3200 CL14 1.45v.
On stock timings and 1.35v it will run fine at 3600 Mhz. 3600 CL14 takes 1.4v, so it could be possible for a CL16 3600 kit to reach this.
The problem is this kit was bought when 3200 was the fastest Crucial made, so its unlikely to still be possible as I think these same chips have now become binned for the 4000 MHz CL18 1.35v kit.
Other things I've read about is that you need to have a daisy chain board and not t topology, and only one channel populated to get the best results. In most cases the motherboard and IMC hold ram overclocking back.