How much cache is in this NAS? Does this NAS unit allow you to adjust the cache allocation? You may need to test the reads multiple times to see a benefit from caching and you may also want to test a write file that's larger than cache to see what happens when you exceed the size of the cache.
Anyway, it is fine for practical purposes and faster than a single drive for sure.
The only real complaint at this point is the fam noise. The fan module does not seem to be serviceable without some surgery that may void the warranty already and the replacement does not seem to be sold anywhere.