1). It could, especially if it is a data warehouse. For OLTP it is less likely to cause a problem, but it just depends on your physical I/O rate.
2). It might, but mainly because you could assign them to different bufferpools.
3). Make no difference. Tables are joined in the bufferpool, not on disk. If a join required physical disk I/O to place the data in the bufferpool, then it "might" benefit more by having them in separate tablespaces, but you might not notice the difference.
4) No, unless you are doing that to control which data goes into which bufferpool.
5) It might help if you are doing multiple of these at one time.