More worst case scenario testing. I have in my possession now a gtx780 and an r9 290. Siege of Shanghai is a great map for showing CPU limits in BF4 under direct X. Server is 64 players conquest.
i5 2500k @ 4.5ghz
Windows 7
Same settings for all cards.
1920x1080
All settings to lowest values
Filtering 16x
Textures to Ultra
Mesh Quality to Ultra
HBAO/SSAO Off
The gtx780 is running at 1176mhz and mem at 6502mhz(highes stable overclock on stock bios), but is downclocking because the CPU isn't feeding it at this location.
R9 290 Tri-X OC @ stock clocks (1000mhz core) in direct x 11 I didn't have afterburner installed when I took these so I'm not sure if clocks were maxed in this screenshot or not.
R9 290 Tri-X OC in Mantle
I'm not sure how much the skyscraper being down effects performance. All I know is that the gtx780 and r9 290 are hitting the same CPU bottleneck in this location (lowest performing spot I could find on the map). LOD between the amd and nvidia card seems to be a wash in dx11. Some details missing that are in the other screen for both shots, and this may be due to texture crawling because of my lack of AA.
I'll will try later to get a shot in mantle of the same location with the skyscraper up. It should be noted that there are 2 tanks, and more players visible in the mantle shot so it should in theory be more taxing than the two dx11 shots.
Mantle nearly doubles my minimums in this location. It should also be noted that even when dx11 is running at 120fps mantle has a definite perceived smoothness advantage. Something about BF4 also makes anything below 90fps @ 120hz nearly unplayable. It introduces massive amounts of input lag and stuttering.