I think you may be correct however Apple takes a ridiculous stance on low noise at the expense of hardware safety. That can be easily rectified with a 3rd party app called "Macs Fan Control".
Also here's a link showing that the aforementioned M295X has a 250W TDP which is crazy for a "mobile" GPU. Since the desktop 980 is 185W I'm going to assume this alleged 990M will be at that TDP level or lower which would be super easy to handle compared to a M295X.
http://www.techpowerup.com/gpudb/2587/radeon-r9-m295x-mac-edition.html
I would assume the same way they cooled the GTX680M & GTX780M & the current M295X in the past 3 years of 27" iMac's.
All of those where the "flagship" mobile GPU's & put out the most heat in the mobile line-up when released & sold in the corresponding iMac.
Because nobody believes they will get hacked. Think about every victim interviewed regardless of the crime & they all say "I never thought this would happen to me, this town/community, etc....