I'm not sure what the current situation is, but at least a few years ago my experience with both Nvidia and AMD was that when upgrading past a certain driver version, fps would tend to decrease again.
Due to the large number of architectures and models churned out by AMD/Nvidia, when releasing drivers, they tend to concentrate on the performance of the latest and greatest cards, and throw a bone to the previous architecture. Older cards are absolutely neglected and may have perf regression after upgrading drivers.
