When we setup a 5,000 user Exchange environment we broke the exchange stores into many stores with no store going over 500GB(for recovery/HA purposes), we actually found the best performance was no more then 6 SCSI disks(VMDK's) per paravirtual SCSI controller. As far as splitting it off I don't think you would get anymore performance unless you have more then 6 drives, however since you have 5 you could split it out with 1 more controller and you might see some small gains.
Hope this helps.