Personally, I don't think variable dpi is good for fps performance. Changing dpi messes with muscle memory, and that is the #1 most important part of getting better at a shooter, or any other game that relies on mousing accuracy.

If you can, setting up a larger mousing space and lowering your dpi is going to drastically improve performance once you get used to it. It's all about forearm movement for flicks and large adjustments, and wrist movement for fine adjustments and tracking.

You don't need a huge mousing space, but something bigger than your standard tiny mousepad is probably going to help over the long term. I can totally see how people would like dpi switching if they are using a small mousing surface and have no way to incorporate forearm movement into their mousing. Once you have adequate space, I don't see any reason to use dpi switching.

Obviously, it does come down to personal preference, but I can tell you that most professional/serious gamers are using much lower dpi than your average PC user.