I managed to find one of these really nicely boxed 16GB kits of G.Skill TridentX. They are PC3-21300 2666 MT/s DDR3 sticks with CL11-13-13-35 timings.
Haven't tinkered around so far but I can confirm that they are functional and stable with my i7-4790K and the ASRock Z97 Extreme4.
How far will they go? Not sure, but I'll try it out!