I had a 900. A aftermarket cooler on the GPU. The side fan did help on temps. It was Yate Loon medium fan.
The one at the bottom could help if you have cards in the lower slot, but it doesn't really increase case flow a lot. The side fan blowing in IS a good idea.
Diff in temps can be so objective depending on fans, what you use as cooling systems etc. It's way to diff to know exactly for sure.