I used mine at 4k and 100hz as well, tracking seemed great on it. The two side buttons are placed higher up the mouse near the very top and it made the grip odd for me. so its now in the pile with a basically new zowie fk as welll that have next to no use lol. I'm liking the ec1 evo cl I got though. how does the ec2 compare to the rival in size or ec1 if you have used one.
Right now I own the Rival, Zowie EC2 EVO and Zowie FK.
Tracking wise, they are all great, I can't really tell the difference between them. So basically it all comes down to build quality and what kind of grip you prefer.
The EC2 is an awesome mouse, the size is just right... not too small and not too big. It's versatile and you can do all sorts of different grips on it, claw or palm or whatever.
The Rival is a pretty big mouse to me after using the EC2 for so long. It's a bit heavier as well but I'm slowly getting used to it. Not a big fan of the side buttons on the Rival though, they are made of a cheap feeling plastic.