Frame pacing by it's nature reduces FPS to some degree. You can't pace frames if you can't average them out, that means delaying some frames to change the difference. It's generally the whole point, but I don't know how much it's hurting performance in this particular situation.
Either way, Mantle across the board has been showing smoother frame rates, lower frame latency and less spikes(outside of some rarer ones in BF4 which I think is BF4 hitting memory limits which they've suggested they need a rewrite of the memory management to work with complete control, the consequence of Mantle being available way after the game was mostly done).
One of the reasons is, and pretty much every dev complained about this on DX for years, is that DX does what it wants. That means sometimes it will randomly compile stuff out of order and cause a latency spike before it can do the next thing.
On consoles in general and now Mantle, and most game design you work to a latency. You can precisely tell how long each part of a frame building takes, how long each part of the engine will take to process and you can design an engine to use say 8ms of compute per frame, then start adding on extra processing effects that add on a certain amount of latency. But these things are predictable. For instance TressFX added something like 4ms of work, and that in TressFX has been cut to 2ms or something, so effectively halving the performance hit for enabling it. Games are a series of things adding up to a certain amount of time each frame. When you can predict the precise order of execution you can control the latency every frame with no surprises.
DX is a black box, it does effectively what it wants after you send it data, latency spikes around as it does things.
A high level API that hides control will always be less predictable and have such spikes. The reason consoles often target an FPS is because precisely that they are targetting a x ms worth of work on each frame and know the exact hardware. The reason frame rate is more variable on consoles is because they can't control the ms per frame.
Mantle, and likely DX12 will always, always be smoother than DX11(other high level api's and all previous version).
