IOPS, as in disk IOS, is not a particularly good measurement of a NAS system and is much more relevant to block or SAN systems.  One of the big reasons is that in a NAS environment, the clients don't really generate IOPS directly.  They make calls via a protocol which eventually map down to disk IOPS, but there is not necessarily a one-to-one or even a one-to-X where X is a constant ratio that makes IOPS a good indicator of performance.   Compare this to block systems where the host sees the storage as a disk and everything is a read or a write.  Reads are, essentially 1:1 and writes are 1:N where N is constant and dependent on the RAID type.  Host IOPS are easier to measure and translating them to disk IOPS is a straight-forward process.  With NAS, we have more than just data read and write operations because we have things like metadata operations that people don't always count as throughout but still happen.  A classic example:  Have you ever tried to copy a directory with a ton of small files and you notice how the throughput stinks compared to a few large ones that constitute the same size?  That's because the small file case has to do metadata stuff and that becomes a significant part of the work but it doesn't always count with respect to "data moved".  This is why you don't see standard NAS benchmarks (of which SPEC SFS is the most popular) talk about disk IOPS.  They talk about protocol IOPS and then they define the exact mix and sizes for the benchmark.  This makes it possible to do a reasonable comparison between NAS systems because you have done a good job of fixing everything else in the environment.  So when an NAS system is rated for IOPS, it typically means protocol IOPS and, more often than not, it means specifically the SPEC SFS workload.  If it is not SPEC SFS, then the workload needs to be well-defined for the benchmark to have any useful meaning.  So to loop this back to your question.  Another commonly seen performance metric used for NAS system is throughput.  Unlike disk IOPS, throughput can be measured from the client and the server which is why you see thinks like X GB/s.  As far as numbers for an arbitrary # of nodes, I would work with you Isilon SE to look at a given config.  They have internal tools that can show throughput for a single stream (to one node, which can vary based on cluster size which makes sense as that changes the # of potential disks) as well as aggregate throughput for protocols such as NFS or SMB (which is another consideration as the protocols differ with respect to performance as well).  This is probably a long-winded answer that doesn't give you a simple number and for that I apologize, but my goal is to get you a meaningful number and like much of our industry, the answer is "It depends...."
The Isilon literature only says that by using the S nodes, ‘up to 200 GB/sec are possible’.  This would only be if you had the biggest system possible.  I can’t find IOPS information at all either ,a way to determine this.  It would be good to have statistics on a minimal 3 node configuration of S nodes and the same for X nodes.  Is there a chart of all nodes IOPS?  Thank you,  Saeid
Isilon Nodes IOPS