Right, after the last hour of messing around, here are my MANTLE 14.2 conclusions.
I run Two 7990's in Xfire. Running 5760x1080. Now, in DX11, I can run ULTRA settings, 2xMSSA and HBOA and it's all pretty darn nice... usually in the 60fps region (apart from Shanghai which is mental in places getting as low as 20-30fps).
Now, with Mantle, as LTMATT stated, it uses tons of VRAM so... messing around with settings noticed this MAJOR bug. If you alter any settings ingame you seem to jump and use around and additional 600mb of VRAM and it won't come down (from circa 3.5-3.7gb)... sometimes it temp locks screen... all slows to 1fps etc... who knows... might just be me, but I have to alter and come back out of game.
Also, I found that running MSSA/SBOA/HBOA in Mantle now breaks the 3GB GPU RAM limit and runs around 3.4GB of RAM, meaning I get CPU spikes all over the shop which then means GPU spikes = choppy gaming. So, left everything on ULTRA and turned these options off, and I max on ANY level 2.95GB of GPU RAM keeping me under my maximum resulting in nice smooth gameplay. result HOWEVER, my system is capable of running these options maxed out IMHO and I prey that AMD reduce this usage of RAM if possible so I can turn them back on.
As for the smoothness... well I think that frame pacing is now working at high res as it does play really smooth... whether that's a combo of frame pacing AND MANTLE working nicely together who knows... BUT it is deffo smoother than DX11. The one thing that's at the back of my mind, I was playing GULLIN PEAK and I swear that the graphics don't seem quit as crisp under mantle as they do under DX11... think I may need some screen shots in the same places to see... might just be my mind lol
EDIT: I've just messed around with VSYNC qwhich I normally don't have on. Not, apart from increasing the CPU to around 16ms... it's smoothed everything out for obvious reasons... so, running around new Caspian, getting CPU around 16ms and GPU's at 32ms... with good stability. The game plays like an absolute dream with GPU VRAM maxing 2,964mb... so keeping under 3gb... perfect...
Am I impressed with Mantle after a couple of hours checking stuff out, yep indeed I am.. it's giving a petter, smoother more consistent game although Shanghai is still fubbered IMHO!
EDIT2: Anyway of showing FPS in Mantle/BF4? Seems like it ignores "Render.Drawfps 1 " now?!?
EDIT3: APPCRASH (AMDMantle64.dll).... do get this every 25% ish of game loads....