Of course that means the data needs to be decompressed to be read so if you try to access the data on the NAS directly, or your Storwize appliances bights the dust, you'll see all the right file names but the contents will be compressed gibberish. Storwize will be glad to sell you a second appliance so you can make a fault tolerant cluster which will solve the problem for production environments. They also have a client decompressor for the truly paranoid like me that worry about what happens when their vendors go belly up.
I'm having a bit of trouble wrapping my head around the 15:1 data reduction claim and the fact that they use the same standard LZ type compression algorithms that never quite lived up to tape drive vendors claims of 2:1 compression. Hopefully I'll get a box in the lab to play with and prove the point.
I don't have any concerns about performance. Compressing the data reduces disk I/O, and network I/O between the NAS and Storwize but not between the Storwize and user as the data's decompressed by then, fewer disk seeks means faster performance even if the compression appliance does add a small bit of latency.
Storwize's compression is applicable to production file systems as well as backup stores and is an especially good fit with NetApp's deduping technology which doesn't do compression.
The question is with CPU cycles to spare why don't NASes compress data? Windows can and I've usually seen performance boosts on Windows file servers when NTFS compression is turned on.