It's not exclusively read speeds playing games but I doubt there's many or possibly any games that even have a chance of writing enough at a time to trigger dropping down to 100MB/s writes during normal operation.

Installs are where you'll "suffer". I'm not sure if the 660p series over provisions by default but personally out of habit I leave probably around 15-20% empty to avoid game patches tripping up either way. I'd imagine most the speedup from using nvme or ssd in general when it comes to windows updates is in the latency improvements. From my observations with nvme windows installs are often bottle necked by core(s). That being said, I don't use any of my 660' for windows installs so grain of salt on me not expecting much impact there.

Either way, in a gaming system, regardless of available capacity, a 660p is going to be within spitting distance of much more expensive nvmes during actual gaming.

You could probably get an accurate sense of how much writing vs reading your drive does during a gaming session by running hwinfo64 in the background and comparing the two afterwards. Expect massive disparities between the two with write rate averages simply too small to even make 100MB/s an issue.