1- Does mean that u want waste ur money, go with 1600 since the difference between 1600 and 2000 is <1%. When u install the RAM, that only can run at 2000MHz not a 4000MHz as u say.
2- XMP is a pre-overcloked RAM. In other words, are RAM profiles to OC depending of ur needs and profile that u select.
3- The better is High speed AND lower CL.