Faster speed and lower CAS is better. If you have searched the web, I'm am sure you will see posts and youtube videos where the author shows you he / she has conclusively proved that faster RAM doesn't matter in gaming by running benchmarks in 3 games. The truth however is that moist games are GPU limited, some are CPU limited and some others still are RAM limited. What also shows up is that a game might not be impacted with a single GFX card but suddenly when in SLI / CF, RAM speed now has a significant effect.
As far as overclocking, the RAM quality may have an impact and it might not. When there is a difference, I try and use Mushkin Redline RAM whenever I can grab it. It's popular with competitive overclockers and it's not because of whose name is on it, but for the modules they use. When you bought 2400 RAM from Gskill, Mushkin or Corsair when that speed 1st dropped, they all used Hynic modules.... as time went on and yields improved, Gskill and Corsair switched suppliers and Muskin stayed w/ Hynix. The Redline series is known for being able to take higher voltages than most sticks which can lead to more stable overclocks.
Here you see the Mushkin 2800s giving even 3000 kits a run for their money.
http://www.tomshardware.com/reviews/mushkin-redline-frostbyte-994206f-ddr4-2800-16gb-quad-channel,4069-2.html
but in the end ... if ya pick two sets of the same spec, more often than not, you will see pretty close to identical results. So while I have my preferences, if I can't get what I like best, I don't have a lot of qualms about grabbing a set of Corsair or Gskill. If aesthetics are in play, I really like the looks of the Avexir stuff.
And no, the speed of the RAM shouldn't affect your OC.... how much voltage it can take might, but that's only going to be an issue at higher speeds than 2133. You should have no problem getting 4.5 Ghz w/ any set of RAM.... if you were going for 4.8 at 3000, might have to settle for 4.7 or a lower RAM speed.