They didn't test mantle themselves. So they were told by AMD. Where else would they get the information, AMD told them 45% and then they say they were told it was more in line with 10% in GPU limited situations. Of course you can twist the words in the article however you want, much like some AMD fans indicated that EA/DICE wasn't paid 8 million dollars for Mantle When 2700+ google search results seem to state that they did.
You can hope for 45%. That's cool. But the chances of that happening, except in excessively cherry picked situations which won't happen in real world gaming? I don't know. Kaveri's pre-launch hype doesn't exactly inspire optimism either. Like you said. AMD have lied before. Right? AMD marketing slides are purposed for hype. Not truth.
Anyway, i'm just saying, 45% is probably not real world. If it's 10 % performance gain for free, if that's what happens as per hardware.fr, that is still respectable.