I'd be concerned if I didn't know the IO density of the applications running on the storage.
Flash cache can make a significant difference for you though.
The IO density of SATA drives is much lower than that of SAS drives.
The factors that account for it are max random IOs per drive, and size of the drive.
A 600 GB SAS drives will do ~175 random IOs, and has a usable capacity of ~500G.
The IO density is 2 X 175 IOs, or about 350IOs/TB.
A 2TB SATA drive will do ~75 random IOs, and has a usable capacity of ~1.5TB.
The IO density is .66 X 75 IOs, or about 50IOs/TB.
Best practice is to monitor aggregate performance, and the IO performance of the applications storage is provisioned for.
I hope this response has been helpful to you.
At your service,
Eugene E. Kashpureff, Sr.
Independent NetApp Consultant http://www.linkedin.com/in/eugenekashpureff
Senior NetApp Instructor, IT Learning Solutions http://sg.itls.asia/netapp
(P.S. I appreciate 'kudos' on any helpful posts.)