the docking station isn't the limitation. the W520's NEC/renesas USB 3.0 controller only has two channels and both are used up by the W520 itself. the dock would need a separate USB 3.0 chipset to meet your request and there's no channel to connect it.
an external USB 3.0 hub might be an option but you'll be limited to a maximum of 500 MB/sec sequential transfers through whichever USB 3.0 port is used on the W520. for most users this would be a non-issue.
On top of that every Thinkpad that is docked would then require a separate driver to be installed for the USB 3 Card to work, which would create complications.
If the series 3 dock was made just for the W520 it might make sense, but the series 3 dock is compatible with just about every Thinkpad under the sun, so adding an external USB 3 card into the dock would have made no sense considering half the Thinkpads it's made to work on have no USB 3 support to begin with.
From my experience USB 3.0 is all hot air, It doesn't even saturate Sata 2 speeds, so 500MB/s is completely out of the question. USB 3.0 currently has lower transfer speed then eSata.
I tested the W520s USB 3 vs the Esata on an identical 8 Bay RAID Array.
The USB 3 maxed out at 180MB/S transfer speed.
The Esata maxed out at 250MB/S transfer speed.
So all this talk of maximum 500MB transfer is all theoretical mumble jumbo, it's about as useful as a multiverse quantum computer.
USB 3.0 in it's current revision is SLOW.