AMD apparently posted developer documentation for an upcoming "Family 15h Models 30-3Fh" server processor. 8M/16C on a single die:
http://www.techpowerup.com/196952/amd-readies-16-core-processors-with-full-uncore.html
"Family 15h Models 30-3Fh" corresponds to Steamroller in AMD's other documentation. The fact that they are designing a new die, and trying to fit in 8 modules (which wasn't done on 32nm - they had two dice on-package connected with MCM) seems to hint that this will indeed be a 28nm part using Steamroller cores. Server parts already usually run at lower clock speeds than client parts, so the clock speed deficit of 28nm may not be as big a deal on the server side.
http://www.techpowerup.com/196952/amd-readies-16-core-processors-with-full-uncore.html
"Family 15h Models 30-3Fh" corresponds to Steamroller in AMD's other documentation. The fact that they are designing a new die, and trying to fit in 8 modules (which wasn't done on 32nm - they had two dice on-package connected with MCM) seems to hint that this will indeed be a 28nm part using Steamroller cores. Server parts already usually run at lower clock speeds than client parts, so the clock speed deficit of 28nm may not be as big a deal on the server side.