GCN 1.2 parts (R9 285/Fury X) have not received any performance optimisation in Mantle titles.
If you take a look at the Thief benchmark
thread you'll see my Quad Fury X's (GCN 1.2) are not much faster than my Quad 290X 4GB (GCN 1.1).
4x GPU's
#1 AMDMatt: Fury-X @1125/500, CPU 5960x @ 4.9Ghz - Mantle FPS: Min 89.1 / Avr 133.7 Link
#2, Kaapstad: GTXTitan-X @ 1442/1952, CPU 5960X @4.5Ghz
DirectX FPS: Min 60.7 / Avr 129.0
Link
#3, AMDMatt: 290X @ 1235/1625, CPU [email protected] - Mantle FPS: Min 38.8 / Avr 120.6 Link
#4, Kaapstad: 290X @ 1230/1625, CPU 3930K @4.8Ghz - Mantle FPS: Min 90.9 / Avr 110.1 Link
#5, AMDMatt: 290X @1200/1625, CPU [email protected] - DirectX FPS: Min 31.3 / Avr 94.1 Link
#6, Kaapstad: 980 @ 1492/1903, CPU 5960X @4.5Ghz - DirectX FPS: Min 30.8 / Avr 90.5 Link
#7, Kaapstad: 290X @1220/1625, CPU 3930K @4.7Ghz DirectX FPS: Min 38.6 / Avr 88.9
Link
#8, Kaapstad: GTXTitan @ 981/1788, CPU 3930K @5.0Ghz
DirectX FPS: Min 37.6 / Avr 85.7
Link