this isnt about doing more with the same - its about doing the same with less.
Only in that case.
But, games were built around the limitations, that's why the speedboost in updated, games designed for mantle will shake the industry with what you call "more".
However some existing games even when designed with Mantle, the genre they're using does not allow to take all that power and turn it into units-on-screen and other stuff because it would not be the the same sub-genre anymore or a spinnoff. In those cases the performance difference will be very visible if there would be a DX or OGL render mode also available, what's more, the minimal requirements would be significantly lower than what would be with DX or OGL version.
For the genres that don't yet exist or have been dead since the 90', if those are designed with pushing the boundaries of AI, physics, visuals, they are not going to have this "boost" effect obviously.
So beware what "mantle reviews" you try, they aren't reviewing mantle API it self which continues to improve and is not a fixed thing so it cannot be reviewed ** , they're reviewing the
mantle code of a specific game or in other words the
effort that a developer put into optimizing the mantle code for that game. Just as with DX games, all the CPU and GPU benchmarks weren't benchmarking the hardware, they were benchmarking the DX API along with driver, so much of the optimization and also stability code residing in the driver basically made all the hardware wars technically invalid, it was and is all a driver war, there was no GPU or CPU war, it's just percieved that way.
Ofcourse practically it can be labeled as "hardware benchmarking" since DX was the only thing used (OGL doesn't even count, a
few AAA pushing-the-limits games here and there) , but it's not a valid technical test at all.
** (eg. pre-release game reviews are usually invalid since day 1 patches became popular, i don't read nor care about any mainstream gaming site)