I used to partition two 500GB drives in my Mac Pro, because I thought it would help with organising different types of files. In the end, all it did was give me more folders to sift through, and I also had to constantly take into account the reduced available space on individual partitions. It was a complete waste of time and effort.
Even on the most demanding projects, I find I never reach drive speed limits. (I use dedicated drives for System, Audio, Samples and personal files such as photos). I'm much more likely to come up against RAM or CPU limitations.