Partitioning can help improve access times and transfer rates IF you partition the RAID volume and use ONLY the first partition. This is because you're basically using only the outermost tracks on all of the drives and:
(a) those tracks have the most data and therefore the fastest transfer rate, and
(b) by only using some of the tracks you're reducing the seek time
If you create multiple partitions and place frequently used files into different partitions then chances are you'll end up slowing things down by forcing the access arms to constantly move back and forth between them.