6GB/s is the bandwith of SATA III I believe, and the current SSDs can't saturate that... meaning there is still quite a bit of headroom for speed improvement until we reach the bottleneck of the transfer cable.
"On the storage front, Apple officially leads the charge with the move to PCIe based SSDs. The upcoming Mac Pro, as well as the new MacBook Airs both use PCIe based SSDs instead of SATA drives. A quick look at OS X's system profiler reveals a PCIe 2.0 x2 interface, capable of 1GB/s in each direction.
The drive in my system uses a Samsung controller, although I've heard that SanDisk will have a PCIe solution for Apple as well. A quick run through Quick Bench reveals peak sequential read/write performance of nearly 800MB/s:...
This is a pretty big deal, as it is probably the first step towards PCIe storage in a mainstream consumer device that we've seen. I'm still awaiting official confirmation as to whether or not this is an M.2 based solution or a proprietary connector. Since there's no PCIe routed off of the CPU in Haswell ULT, these 2 lanes come from the on-package PCH."