As Malus states below, it is sad but it is normal with the Retina iMac, at least the 2014 and a maxed out one. I was getting ~105ºC since day one, these super high temps have been happening more lately.
Apple offers 1 year without Apple Care. Spain law's makes it so they have to offer 2 years, but that too expired last January, I didn't have this issues until like 1 or 2 months ago.
I have been using iStats since I bought the iMac. It lets me monitor temps in the task bar and also allows for fan control. If I set them to max before starting to export, it takes a little longer but in ends up getting at 105 - 107ºC no matter what. Then, maybe, and sometimes, it'll reach more and start the issues.
Haven't tried that actually. I tried a vacuum cleaner accessory that is kind of a tube and put it right in the back grill (where the hot air comes out from), but didn't really seem to get much out of it.
Having a lot of dust inside of it is definitely a possibility, now is when I hate that you can't easily open up an iMac.
Yeah, during summer time I must turn AC ON, no doubts. As for taking it to the Apple Store, problem is I'd be several days without the computer and I need it almost 24/7. Once I jumped to 4K I realized this 5K iMac hadn't been the best choice, I would've benefitted better from a Mac Pro I think, but I can't justify buying a 4 year-old computer at that price point, even with the... "price drops".
My best chance is, I think, wait for this new Mac Pro they're allegedly working on, probably 2018, and hope for the best with my iMac.
Thank you all!
Damn seems like you've already tried all of the standard solutions.
Here are some other, more drastic steps I thought of:
1. Disable Turbo Boost: This will initially slow your machine down a bit (no boosting to 4.4Ghz), but given that the 2014 models already throttle down their base clock of 4Ghz when they hit 107C, this shouldn't hurt performance too much while allowing the chip to stay cooler.
2. Disable Hyper Threading: Given your use case, this could have a more dramatic impact on performance, but may (emphasis on the may, I've never tried this) also help the chip to stay cooler as it will be doing less work.
3. Underclock/Undervolt the GPU: This one's pretty extreme and I'm not sure it's even possible, but if you can get the GPU to use less power that should have a massive impact on the amount of heat generated. Again, I have NOT tried this but theoretically it MIGHT be possible using something similar to the following procedure:
A. Backup macOS and then install the Clover Bootloader (normally used for for hackintoshes) onto a blank EFI partition on a bootable drive.
B. Either via Bootcamp or Clover, install Windows (the latest Windows 10 insider preview will work if you don't have a license)
C. Dump your GPU's BIOS file (various overclocking utilities such as MSI Afterburner/Sapphire Trixxx will allow you to do this)
D. Edit your GPU's BIOS file to lower the core clockspeed and or slightly undervolt the various power states (look this up if you don't know what it means) (Look for a guide on Tonga BIOS editing, and pray that the Mac BIOS has similar values)
E. Save the edited BIOS to the Clover EFI partition (DON'T FLASH IT BACK TO YOUR CARD!), and use the Clover vBios function to have Clover run your graphics card with the modified bios (without overwriting or otherwise modifying your cards actual BIOS)
Just to be clear, while I don't know if the above will work (I don't know if Clover's vBIOS function will work with a real mac / whether or not the BIOS Apple has on the card is similar enough to the desktop R9 285 for any PC BIOS editing guides to be applicable) there shouldn't be any serious risk to the machine as you're not actually modifying the BIOS file on your graphics card. If you've never built a hackintosh or messed with GPU overclocking/bios editing on Windows, its probably going to be a lot to take in. That said, if you're interested in giving it a shot I can point you in the right direction.
4. Edit the AMD Graphics Driver Kexts in macOS to disable GPU compute units: This is a slightly less extreme than #3 but the principal is similar. The eGPU/Hackintosh community recently figured out how to modify the macOS AMD Graphics Driver Kexts to remove the compute unit (CU) limit on officially unsupported cards (the R9 Fury/R9 4xx series). These cards were previously limited to 16 CUs (the number of CUs the R9 460, the only officially supported "new" AMD card had), but can now access between 32-64 (depending on the card. While I'm pretty sure the Kext mods currently known only work on the Kexts for the latest AMD cards, if you could find a similar hack for the "Tonga" class of cards (M295X/M395X), you might be able to disable a few CUs and reduce the heat output (at the cost of performance). Again, I can point you in the right direction if you're interested, although it's probably a bit of a long shot.
5. Get an eGPU and an External Monitor: This will be expensive, but should allow you to relieve the strain on the iMac's internal cooling system. That said, Final Cut doesn't always play well with multiple GPUs (outside of a real cMP/nMP), so you should definitely check with the eGPU community first for advice. Also, I believe using an eGPU with the 5K iMac requires a hack which limits the resolution of the internal screen to 4K (although this may have been resolved, I haven't checked lately)
Anyway, food for thought. I'd really recomend you do whatever you can to get those temps down, otherwise your iMac may not last until the nnMP/2017 iMacs....