The Leatrix Latency Fix will not have any bearing on your FPS (DPS yes, FPS no).... your connection to the net plays no roll on how your display driver renders what is on your screen.
It is safe to delete the Cache, Interface, WTF, and Screenshots folders. Typically it is not necessary (or advisable) to delete any other folders in the games root directory. As for what's inside of the Patches/Updates folders, I cannot advise on that as I've not seen any of the techs on the official forums say to delete anything in there (not saying you can't, just saying I've not seen them say to do it). Personally, I leave em alone.
I can't really tell you what you need in your Config.wtf file as I'm not sitting in front of your comp to test things (and the suggested one on the tech forums is extremely old). I can, however, recommend that you read this sticky thread;
System Performance Guide, 3.x edition.