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The card in your link has open dual fan design, not good for a Mac Pro. Prefer reference blower design, and you will have no issues with heat.

P.S.: Avoid all 'classified' cards with 2 * 8 pin, if you want to use it in a Mac Pro.
 
I am running an EVGA 980ti ACX 2.0 in a 4,1 along with a maxwell titan x (blower) using slots 1and 3, powered by pixlas mod. No heat problem with open fan design nor blower design. Fans do spin up of course when rendering, but the Macpro manages well.
 
The PCIe ambient temperature may go up a lot if you use the open fan type card, the hot air will be a bit hard to get out because there is no proper exit for the PCIe compartment. However, I don't think it can really cause any issue. The 980Ti rarely killed by heat. Its fan can spin up a bit more, and thermal throttled if really too hot.

Also, the hot air can't stay, the PCIe fan still keep pulling cool air in, eventually the warm air either "leak" out, or exhaust by the PSU, or pulled out by the exhaust fan, until the PCIe ambient reach the new equilibrium temperature.

If noise is not a big issue, I believe blower type reference card should be the good choice. PCIe ambient will still go up a bit, but much less than by the open fan type card, and the airflow is very straight forward to follow the Mac Pro's design.
 
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