There is only the briefest mention in the Huawei docs to hand, but from what I see, the backplane remains unchanged. It's a dumb component anyway in the MA56xx.
From the MA5616 Hardware Description - document reference V800R310C00 :
"An MA5616 supports common and enhanced subracks. If an enhanced subrack is used, the
control board, service boards, and software versions of the MA5616 V800R311 can be used to
support the vectoring function."
I take it to mean that the "subrack" is the cabinet or rack enclosure that holds the MA5616 chassis. Huawei sells such a beast - "an enhanced subrack" although why a different rack should be needed for vectoring?!
Could well be wrong though.
It looks like it employs a system level vectoring architecture. Though back in 2009, Huawei was prototyping node level vectoring with two or more cascaded MA5616 chassis. 
Edited by asbokid (Wed 24-Apr-13 12:36:40)