I had the same experience about throughput being a bit sub-par. For me it was a test on a single macbook pro with a regular 2.5" hdd.
Which client did you use to write to riak? protobuf or http? Also: which language? did you use threading? Did you enable search?
Well, for the benchmark, I was using Basho's benchmarking tool which is erlang, and I was testing with protobuf. I had 5 concurrent clients running for the benchmark, but also tried with more and less, and got about the same results.
