Going through NetApp sales team will give you a better answer. They can size it to give you the best educated recommendation.
That been said, here is some ideas to think about.
If you are adding ESX, Exchange or SQL data, i think adding more SSDs will give you a better performance.
If The cluster AFF is running High on cpu, then maybe add a FAS pair is an option.
If you are adding cold data or if the cluster will have High amount of cold data on the Vservers, specially NAS Vservers , then tiering to the StorageGRID will even be a better option.