The mSATA drive is using the miniPCIe connector for power, but is using only the embedded SATA lane for data communications (i.e. it is NOT using the PCIe lane(s) available). In modern systems, this communication is limited by the 6Gb/s capability of the SATA lane. This means that the maximum transfer rate is 750MB/s, but due to some physical overhead that occurs when encoding data for transfer, it actually has a practical transfer rate of 4.8Gb/s (600MB/s).
You're right, unfortunately. I keep forgetting that it wasn't until the 8 Series chipsets that they had SATA III 6Gb/s support across all 6 lanes. It is a SATA II lane that they routed to the mSATA connector (see diagram on page 17 of the TPS). That was a poor decision, if you ask me; this means only 3Gb/s max transfer capability.