"I wonder why AMD decided to do 1M and 512K parts separately."
The original plan was to have 1M and 256k parts. But with the delays and the delamination problems, they went with the 1M initially, and added 512k with dual channel. Apparently the 256k parts got resurrected for Paris/Dublin.
I would think that a 256k, single channel part would have decent performance. But marketing considerations are also important, and it might have been deemed difficult to get much of a premium for such a part.