I have a DNS-323 since it first became available, I've used it at both 100 and 1000 speed links, I've tried all possible RAID (or not) configurations, and my conclusion is that if you are seeing speeds around 15MB/s you should be happy. I am usually getting between 11 and 18 MB/s, depending the RAID config and all other circumstantial factors.
Also, if you still have it, check the disk that came with it, and read the manual. The manufacturer claims about the same transfer speed, so we are all good, let's not keep beating this old horse, it ain't dead yet but it won't give you more than what it was designed too. It's more than 10 years old and in computer technology this is the ice age.
Having said that, it is unlikely that those who report speeds around 30MB/s did a proper testing, or they are either NOT telling the truth or they don't know what they are talking about or all the above. So take these claims with a grain of salt, or just drink a beer and relax. Life is too short to worry about the DNS-323's speed.
Cheers mates!