I apologize, you wrote NAS but my brain said SAN.
I cannot say what performance you should see, but, with 1600 clients you should easily not see too much overhead. It all depends on what you are doing with the server.
Do you have at least one site server running Task Services? If not, then I would have to assume your NS is also servicing tasks. This will absolutely kill your Notification Server. So if Task Services is not off loaded to a site server, consider that.
How much memory is on the server?
Finally the disk subsystem on the server is important. I would like to refer you over to an excellent article by ianatkin, located here: https://www-secure.symantec.com/connect/articles/getting-hang-iops
The important thing I would try to suggest is that everyone is setup differently which could dramatically change your experience with the product. Person A could have 1600 clients and could see great performance, while the next person has terrible performance.