If the fans are blowing out of the case remove the fan and find a better place.
If they are blowing into the graphics card and the case fan is slower keep it. If it is alot faster (which is highly unlikely) remove it as it might damage the graphic fan bearings
your reasoning int entirely correct. It might be that it is slower due to more force needed to spin because of the air from the extra fan pushing the original fan in the other way than it is spinning.
That's correct. But in my case the casefan is blowing in the gpu's direction, plus it's sgnificantly slower. I've done some testing and figuered out that the casefan, although spinning slower, provides some extra cooling for the gpu.
I assume that InCoinsITrust came to the conclusion, that my casefan is dragging the air out of the case, but it's actually not. Otherwise I don't see a reason for a fan blowing in the gpu's direction would harm the gpu-fans. UNLESS it's spinning faster, which is (sub-80 is right about that) is unlikely.
This. I ran 12-13 pc's with 23 gpus back when mining for btc was all gpu For about 14 -16 months.. Fans blowing onto gpus fans are fine as long as they don't overwhelm the gpu fan.