I would say it is probably working. The statement about minimum cards is what they officially say they support. Generally it should be working with much older cards. However, it might be not very benificial to use it with those older cards.
I have a GTX 970 which is not officially supported too, but I think it is working as intended.
What screen resolution are you using?
What quality level or percentage did you select?
Lets assume you use 1080p with Performance preset. That means it uses 50% of the native screen resolution internaly and upscales to 1080p. Unfortunately these are pretty few pixels for the upscaling algorithm to work with so the result is not the best.
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FSR will deliver better results when it is applied in situatations where the resolution is allready quite high. It seems to be really good if you target 4K. 1440p is kind of a middle ground. There it depends pretty much on you if you are willing to trade some loss of image quality for performance. I tried 1440p with balanced. I'll probably stick with that because it gives me 10-15 FPS (so I'm now around 60) but I'm sure a lot of people would not be using it.
Please correct me, if I got something wrong.