I believe there's a rule that you shouldn't go beyond 1.6k, but I could be wrong.
Anyway, I'm oldschool, still using my IE 3.0 (old white version). This mouse is indestructible, had problems with scroll but fixed it and fingers spots wear off but other than that it's golden. I own it since 2004. I used to play competitive, but not anymore, not really that much interesting games for playing online to be honest.
But I saw Logitech G500 and MX518. G500 was too small for my hand, MX518 was pretty good in comfort area, but I still think IE 3.0 was comfier, but I didn't really play on Logitech so no opinion. About Razer, to be honest I mostly define their products as crap, I do think they had two good products: Razer Boomslang and Razer DeathAdder (first version). I heard that they fixed the drivers for new version of Deathadder so there might be hope.
Generally I am disappointed in what direction the companies producing mice went on. There is a mouse that has 10k DPI, I mean come on! I actually wrote this post before reading whole thread, so I decided to read the 5 pages and see what mice people here are using. I really like that Logitech company, there's no marketing ******** like with Razer or Steelseries. People really should look what sensor is inside the mouse, the same sensor that sits in Logitech/Razer you can find in 3x cheaper A4Tech, but ultimately it comes down to functionality and comfort zone.
Perfect mouse gaming would be the housing of IE 3.0 with fixed buttons, wearing off and scroll + sensor from old Razer Deathadder... Although, I am intrigued by Steelseries Xai and Sensei.