I'm looking for a servo for my 4wd buggy and I looked at the 7955tg and it comes in 3 diff mods...90..180...and continous rotation...is that something that hurts or helps performance of a servo??

180 is standard which is what you really need for cars. The others are perhaps aimed at specific applications.

I don't know of any use for continuous rotation in RC. It's a special application for things like photography and robotics.

90* is stock for this servo. 45* each direction.
Continuous rotation is also used for sail boats as a sail winch.

Yes on both points. The higher angle option sacrifices resolution. My bud has a sailboat with one, it's a lot of low stress fun!