I didn't notice much difference with Mantle in BF4, from a moothness point of view anyway.
Framerates were higher though that's for sure. Well, for a few minutes anyway and then they dropped to single digits when the VRAM was exceeded. The only way to get around it was to turn the settings down. I don't think that was all that noticeable either though. Just annoyed me that I had to reduce setting from what I could use with DX. I reduced the settings in Mantle and it performed better than DX did with higher settings. Seems a bit obvious really...
That said Mantle was introduced to BF4 so late that I'd finished the game by the time it was added so I didn't get much of a chance to really try it other than for the sake of trying to test Mantle. And then they didn't add Mantle into any other game I was interested in.
I'm hoping DX12 and Vulkan will be so I'll get chance to use it in games I want to play.
So is there any chance you'll decide against getting 2 Titan Xs and instead go for 4 x 8GB 290Xs and save some money Greg?
I have not overly played any game with the 290X, so every game I have mentioned (except ED) has had a quick 20 minute play but I don't foresee any issues.
As for changing my mind on the TX, if AMD can get me 144Hz on my Swift, who knows. I am always up for saving some cash but I wouldn't go 4 cards. One thing I noticed with ShadowPlay (something I use a lot) is how it works very well with 1 or 2 cards but add in a third card and basically, ShadowPlay took all the extra performance that the third card offered and gave me the same frames as what 2 cards did and even if I ran 2 cards with ShadowPlay running, I got the same frames as I did with 3 cards and ShadowPlay running.... Not something to fuss about but made the third card pretty pointless for me.
I hope that makes sense?
So in a nutshell, I won't be going 3 cards again and CF or SLI only.