I know that the best thing to do when trying to play fast is to play slowly, and work on the fundamentals of playing, but is this best done by playing at a speed you are comfortable at, or playing WAAAY below this?

you want to push your limits thats what its all about... dont be sloppy use good technique but the second you get sloppy and start cheating youre trying to go too fast.. so to answer your question you want to play that a level that you can keep up with steadily and you will slowly graduate to higher and higher tempos.. hope this helps