I understand what you meant. If the more powerful GPU were just a more better version of the less powerful GPU, the bottleneck would be the limit and all GPUs would have the same performance (bottleneck limited)
However, it is never that simple. The CPU bottleneck limits the data processed sent to the GPU. More powerful GPUs may be more efficient and games are not even in their load on CPU vs GPU, so a game that is very CPU intensive will show little effect no matter what the GPU. Games that are very GPU intensive, even if bottlenecked by the CPU, will show significant improvements anyway, and some operations essentially bypass the CPU.
So, it all depends on the specific game in practice.
Here's a question for you. What are you...