Well it will ultimately depend on the game. You're not RAM speed limited by the Maximus motherboard, just anything above 2133 you'll need to OC.
RAM speed is much more relevant in some modern day games, whereas it never used to be. Realistically in some games. RAM speed has been shown to improve performance by more than 5%. Such as in Fallout 4, Battlefield, F1 etc. But in many applications, there is still little difference.
You see biggest jumps when going from say 1600 to 2400, beyond that, the benefits often are shown in editing programs. But as I said, some games are showing better utilisation of RAM.
So IMO. If you're focusing on a gaming build, anything at 2400 or above will do you absolutely fine. If 3000 is less expensive...