Playing "devil's advocate" if the new 'mono/mantle' API is pushing GPU utilization higher, then logically it will also be generating new heat.
You'd think that, but it's not really the case. Why? Because for starters, A: mantle is only intended/an improvement for PCs, because PC OS has a thick abstraction layer between games software and the actual hardware. Consoles with software APIs (starting with 3D accelerated consoles really) have a much thinner layer due to the fact all hardware is uniform. Every playstation, xbox and so on is functionally identical to each other so there's no need for the abstraction stuff of PCs. Mantle do away with much of the abstraction because AMD knows the architecture of its own hardware.
Also, B: without mantle, programmers seek to limit the number of processor-expensive draw calls by batching together huge number of triangles to maximise GPU utilization without banging into a CPU-limited ceiling. All mantle really does is not enable anything new, but rather greatly reduce the price (in CPU cycles) of a draw call, so that you can afford more of them, in a more flexible manner, without maxing out your CPU cores.
So result with mantle is, you get the same amount of heat, but in a more flexible way.
(there's a similar concern on the PS4 - 'free compute' does not include 'free cooling for that compute').
There's no need for any such concern, as the cooling is dimensioned for the maximum thermal output of the circuitry.
even CBOAT seems to suggest that his rumour won't make any sense until after the launch of the console. (which makes it a rather useless rumour to discuss).
I'd venture a guess and say it's useless and a waste of time discussing it regardless of when you discuss it, pre- or post-launch, because it sounds completely bogus. Someone's getting a kick out of making up a the-sky-is-falling doomsday scenario because those tend to self-perpetuate easily across the web. Negativity always gets a lot more attention, unfortunately. A common human failing, I'm afraid.