Sunday 7th April 2013: 7.23am.
For comparison I threw in a Sandisk Ultra UHS-I microSDHC, and it gets 43Mb/sec reads and 9Mb/sec writes. Note the hard drive like write latency of 8ms or so.
One BIG concern is the random 4Kb writes which are awful even by SD card standards - I think they sacrifice random access for throughput in these newer fast cards. Must admit I can see me turning off barriers, any form of write guarantees etc.
So, on to finding a SD card with reasonable random read and write scores ...