This may not be helpful, but I don't think overclocking a 2070, or almost any recent video card, will change the performance of a racing game to a noticeable degree in actual game play. The only meaningful improvement you would likely find is in benchmarks - as you have seen.
While I agree in general, I don't think this is always the case. Testing of the chips seems to be lacking, like the PSU issues with early 3080 drivers showed and marketing probably has some role in the MHz too.
I gain a few % in ACC, while saving 100W. I find this to be pretty insane.. Forums and reviews all show the same MHz @ same mV that I'm using so it doesn't seem likely that I have a "super chip".
With my GTX 1070 I could gain up to 10%, which isn't just a tiny bit.
But as I said overall I agree. Modern products need to perform stable even when being choked in a dusty case with only the CPU fan in a climate where you have 35°c in the rooms.
That's no comparison to my high airflow build with 25°c max during summer.
I'll have to dig into Afterburner more, I was not aware of the graphs it could display.
However, driving home today, I thought I am probably chasing something that is not possible to do. My GPU is already running at 99% (CPU around 25%), so if it is already at 99%, there is nothing more to gain, correct?
If you read my post again (edit: sorry! I had a word flipped there! Fixed it!), then, as R8pilot said:
Having the graphics card load at 99% is exactly the usecase where overclocking the graphics card will result in directly higher FPS!
If your graphics card is below 95-99% (depends on the application and exact GPU), then running higher MHz will only result in more headroom, since you don't need the maximum performance anyway.
The load value is always in relation to the maximum possible load at the current MHz.
This results in some funny stats like my 3080 showing 50% load while watching a youtube video.
It's running at 510 MHz only though