Since ESX 4u1 you have a new SCSI controller type available for your guests, the VMware Paravirtual SCSI Controller.
From several sources we learn that the performance of this controller is better.
See for example the New !! Open unofficial storage performance thread where Lars gave some interesting performance figures.
And in the PowerCLI Community there is a recent thread, called Change SCSI adapter to paravirtual with PowerCLI, where the user asks how he can change his disk adapters.
Reason enough to look how I could use PowerCLI to migrate guests to the VMware Paravirtual SCSI Controller.