Here is an easy (but still not 100% accurate) way of testing it:
When the in game memory amount is ABOVE the card's actual installed capacity, check the GPU usage. If it is still at 100% then it is likely the GPU core itself chugging along. If usage drops drastically (say to 75% and lower), then memory bottlenecking is a distinct possibility. This is because the GPU will throttle itself if the memory can't keep up with its dispatch requests.
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