Are the SAS and SATA disks owned by the same controller? If so this is most likely your problem. When doing writes the NVRAM is shared and the slower SATA disks will affect the entire controller performance, even those volumes using SAS disks.
We have this same issue, best bet is to remove the SATA disks....... or find a way not to use them very much.
It would be interesting to see the same sysstat results when the hosts are doing a large number of writes to the SATA disks. If you are a MS SQL shop try a large DB backup to local disks or snapshot the VM leave it for a while to build up a few GB of changes and then commit the snapshot with sysstat -x 1 running.
Keep all of your VM's on non-SATA drives, regardless of any internal SLA's you may have established. They don't perform fast enough and can cause all sorts of slowness in your filer evnrionment. Base your SLA tiers on resource pools within your virtual management application.
As for NFS, NetApp's run great using the protocol. We have multiple filers utilizing NFS and have no issues with the NFS protocol. If you are using VMware, make sure you download and install the NetApp Virtual Storage Console for vSphere and run the tool to set all of your NFS timeout and settings. Also verify that you have your VIF configured correctly on the filer and that you are teaming it properly throughout your network switch.
MOST IMPORTANT!! Make sure that all of your VM's are properly aligned. Mis-aligned VM's cause a tremendous amount of double work on the filer and can severely hinder the overall performance of the filer. There are a number of resources out there on how to align your VM's along with built in tools in the Virtual Storage Console.
Verify there are no other workloads/flexclones that are running out of control causing extra workloads on your filer.