The maximum scsi controller per VM is 4, the general best practice is separate OS from data. One controller for your OS vdisk and one controller for rest or any high IO vdisk. I would recommend reading virtualizing exchange server best practices, it goes into more details...Here is the link for Exchange 2010 on VMware http://www.vmware.com/files/pdf/exchange-2010-on-vmware-best-practices-guide.pdf
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.