Could the new block be related to part of the audio subsystem that deals with Kinect speech processing?
I guess the general consensus is that the amount of SRAM looks rather big for audio purposes. At least the amount is quite a few times the amounts on the Hotchips slides if I recall correctly.
http://www.eurogamer.net/articles/2013-09-04-xbox-one-hot-chips-processor-presentation
It has four DSP cores with 16KB/32KB I/D cache each and then 64KB SRAM.
I saw someone post 2560KB for the SRAM in the "new" block. Not sure were 2560KB came from but the block looks in that rough range of size relative to the two 16MB (+ ? redundancy) GPU SRAM blocks.
I guess no one really knows at this point.